gh: Initial commit of issue template

ISSUE_TEMPLATE.md

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+Thanks for taking the time to file an issue! If you have general question or a problem with your email account,
+take a quick look at the [N1 Knowledge Base](https://support.nylas.com/hc/en-us/sections/203638587-N1_)
+to see if you question is addressed there.
+
+Our team tries to respond to all GitHub issues. To make sure your issue is
+actionable, try to include the following information:
+
+ - Are there any related issues? Try searching for both open and [closed issues](https://github.com/nylas/N1/issues?q=is%3Aissue).
+   Keep in mind that email features are often described differently on different platforms.
+   (Conversations == threads, shortcuts == hotkeys, etc.)
+ - What operating system are you using?
+ - What version of N1 are you using?
+
+**Bug?**
+
+ - Do you have any third-party plugins installed?
+ - Is the issue related to a specific email provider (Gmail, Exchange, etc.)?
+ - Is the issue reproducible with a particular attachment, message, signature, etc?
+   Try to provide an example as a file attachment or a screenshot.
+
+**Feature Request?**
+
+ - Does this feature exist in another mail client or tool you use?