Add developer documentation to Git

Currently, this only includes some remarks regarding validation.

docs/dev_documentation.md

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+# sciNote Developers Documentation
+
+## Front-end (React.js)
+
+### Validations
+
+**Validation components**
+
+Whenever you have a pretty generic form, use the validation components on the front-end and back-end.
+
+An example use case for how to handle this new validation can be seen on New Team page. The validators are merely a single-input-field validators, and can only get you so far. For more complex validation (e.g. whether password & password confirmation match), server-side validation should be used.
+
+The validation is run by providing the same `tag` property to various elements that relate to the same "data field", as well as hooking the relevant validators to the `<InputFormControl>`, and voila - the validation magically works :rainbow:!
+
+Also the `<ValidatedSubmitButton>` is automatically disabled as soon as there is any error on the form.
+
+To call the form itself from within the hosting component, you should use the `ref` mechanism:
+
+```javascript
+<ValidatedForm ref={(f) => { this.myForm = f; }}>
+```
+
+Additional validators can be added once we need them. They are basically a Javascript function that receives `target` (input element reference) and `messageIds`, and returns an array of errors (which are a JSON object which support localization). This will be better once we onboard flow onto them.
+
+**How to render server errors?**
+
+If the server returns any error (this excludes actual validation errors, mostly related to the input data being incorrect), ideally we want to display this (e.g. `Oops, something went wrong!` or similar) by one of 2 means:
+
+* if it's a form, display it as a general form error feedback;
+* if NOT a form, display it in a flash message (if this is the case and the error occurred in modal window, modal **MUST** be closed, because otherwise flash message isn't really seen by the user properly).