add: contribution guide

CONTRIBUTING.md

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+## Contributing
+
+First off, thank you for considering contributing to Telegram Media Downloader. It's people like you that make telegram-media-downloader such a great tool.
+Please take a moment to review this document in order to make the contribution process easy and effective for everyone involved.
+
+### Where do I go from here?
+
+If you've noticed a bug or have a feature request, [make one](https://github.com/Dineshkarthik/telegram_media_downloader/issues)! It's generally best if you get confirmation of your bug or approval for your feature request this way before starting to code.
+
+If you have a general question about telegram-media-downloader, you can ask it on [Discussion](https://github.com/Dineshkarthik/telegram_media_downloader/discussions) under `Q&A`  category and any ideas/suggestions goes under `Ideas` category, the issue tracker is only for bugs and feature requests.
+
+### Fork & create a branch
+
+If this is something you think you can fix, then [fork telegram-media-downloader](https://help.github.com/articles/fork-a-repo) and create a branch with a descriptive name.
+
+A good branch name would be (where issue #52 is the ticket you're working on):
+
+```sh
+	git checkout -b 52-fix-expired-file-reference
+```
+
+### For new Contributors
+
+If you never created a pull request before, welcome [Here is a great tutorial](https://egghead.io/series/how-to-contribute-to-an-open-source-project-on-github) on how to send one :)
+
+1. [Fork](http://help.github.com/fork-a-repo/) the project, clone your fork, and configure the remotes:
+```sh
+   # Clone your fork of the repo into the current directory
+   git clone https://github.com/<your-username>/<repo-name>
+   # Navigate to the newly cloned directory
+   cd <repo-name>
+   # Install dependencies
+   make dev_install
+   # Assign the original repo to a remote called "upstream"
+   git remote add upstream https://github.com/Dineshkkarthik/<repo-name>
+```
+
+2. If you cloned a while ago, get the latest changes from upstream:
+```sh
+   git checkout master
+   git pull upstream master
+```
+
+3. Create a new branch (off the main project master branch) to contain your feature, change, or fix based on the branch name convention described above:
+```sh
+   git checkout -b <branch-name>
+```
+
+4. Make sure to update, or add to the tests when appropriate. Patches and features will not be accepted without tests. Run `make test` to check that all tests pass after you've made changes.
+
+5. If you added or changed a feature, make sure to document it accordingly in the `README.md` file.
+
+6. Push your  branch up to your fork:
+```sh
+   git push origin <branch-name>
+```
+
+7. [Open a Pull Request](https://help.github.com/articles/using-pull-requests/) with a clear title and description.
+
+
+### Coding Standards
+#### Python style
+Please follow these coding standards when writing code for inclusion in telegram-media-downloader.
+
+Telegram-media-downloader  follows the [PEP8](https://www.python.org/dev/peps/pep-0008/) standard and uses [Black](https://black.readthedocs.io/en/stable/) and [Pylint](https://pylint.pycqa.org/en/latest/) to ensure a consistent code format throughout the project.
+
+[Continuous Integration](https://github.com/Dineshkarthik/telegram_media_downloader/actions)  using GitHub Actions will run those tools and report any stylistic errors in your code. Therefore, it is helpful before submitting code to run the check yourself:
+```sh
+black media_downloader.py utils
+```
+to auto-format your code. Additionally, many editors have plugins that will apply  `black`  as you edit files.
+
+Writing good code is not just about what you write. It is also about  _how_  you write it. During  [Continuous Integration](https://github.com/Dineshkarthik/telegram_media_downloader/actions)  testing, several tools will be run to check your code for stylistic errors. Generating any warnings will cause the test to fail. Thus, good style is a requirement for submitting code to telegram-media-downloader.
+
+This is already added in the repo to help contributors verify their changes before contributing them to the project:
+```sh
+make style_check
+```
+
+#### Type hints
+Telegram-media-downloader strongly encourages the use of  [**PEP 484**](https://www.python.org/dev/peps/pep-0484)  style type hints. New development should contain type hints and pull requests to annotate existing code are accepted as well!
+
+Types imports should follow the  `from  typing  import  ...`  convention. So rather than
+```py
+import typing
+
+primes: typing.List[int] = []
+```
+You should write
+```py
+from typing import List, Optional, Union
+
+primes: List[int] = []
+```
+
+`Optional`  should be used where applicable, so instead of
+```py
+maybe_primes: List[Union[int, None]] = []
+```
+You should write
+```py
+maybe_primes: List[Optional[int]] = []
+```
+
+#### Validating type hints
+telegram-media-downloader uses  [mypy](http://mypy-lang.org/)  to statically analyze the code base and type hints. After making any change you can ensure your type hints are correct by running
+```sh
+make static_type_check
+```
+
+#### Docstrings and standards
+A Python docstring is a string used to document a Python module, class, function or method, so programmers can understand what it does without having to read the details of the implementation.
+
+The next example gives an idea of what a docstring looks like:
+```py
+def add(num1: int, num2: int) -> int:
+    """
+    Add up two integer numbers.
+
+    This function simply wraps the ``+`` operator, and does not
+    do anything interesting, except for illustrating what
+    the docstring of a very simple function looks like.
+
+    Parameters
+    ----------
+    num1 : int
+	    First number to add.
+    num2 : int
+	    Second number to add.
+
+    Returns
+    -------
+    int
+	    The sum of ``num1`` and ``num2``.
+
+    See Also
+    --------
+    subtract : Subtract one integer from another.
+
+    Examples
+    --------
+    >>> add(2, 2)
+    4
+    >>> add(25, 0)
+    25
+    >>> add(10, -10)
+    0
+    """
+    return num1 + num2
+```
+Some standards regarding docstrings exist, which make them easier to read, and allow them be easily exported to other formats such as html or pdf.
+
+### Code of Conduct
+
+As a contributor, you can help us keep the  community open and inclusive. Please read and follow our  [Code of Conduct](https://github.com/Dineshkarthik/telegram_media_downloader/blob/master/CODE_OF_CONDUCT.md).