Add initial contributor guidelines

Author: tomv564

CONTRIBUTING.md

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+# Contributor guidelines
+
+## Before you start
+
+LSP is a universal language server package.
+It is not the place for language-specific logic, features or workarounds.
+
+Sublime Text is not an IDE.
+Sublime Text users may appreciate IDE-like features, but a significant number of users will want to turn these features off due to performance or to keep the package "out of the way".
+
+Please feel free to create an issue before coding:
+
+* If you are unsure your proposal is suitable
+* If you are unsure your solution will be technically sound
+
+The issues also allow you to gather some feedback and help from other contributors.
+
+## Coding
+
+Sublime Text 3 bundles a Python 3.3, please be sure to set up your environment to match.
+LSP uses two flake8 and mypy to provide some code quality assurances.
+I highly recommend enabling plugins for these in your Python coding environment.
+If you have these tools available on your machine, you can check your work with:
+
+```
+flake8 main.py
+mypy main.py
+```
+
+## Testing
+
+Please consider testing your work with other language servers, even if you do not use them.
+The Javascript/Typescript language server is a good example, it has a fairly complete feature set.
+
+## Submitting
+
+Before you submit your pull request, please review the following:
+
+* Any unrelated changes in there?
+* Is it a bug fix? Please link the issue or attach repro, logs, screenshots etc.
+* Is it a feature? Please attach screenshots / GIFs for visual changes.
+
+I will try to help you get the PR in mergeable shape within a reasonable time, but it may take a few days.
+It is best if you check your Github notifications in the meantime!