Adds CONTRIBUTING.md (#319)

* Adds CONTRIBUTING.md

Closes #206 
Refs #274

* Update CONTRIBUTING.md

CONTRIBUTING.md

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+# How to contribute
+
+## Tutorials
+
+If you want to start working on this project,
+you will need to get familiar with these projects:
+
+- [Django docs](https://docs.djangoproject.com/en/dev/)
+- [Typing in Python](https://inventwithpython.com/blog/2019/11/24/type-hints-for-busy-python-programmers/)
+- [How to write custom mypy plugins](https://mypy.readthedocs.io/en/stable/extending_mypy.html)
+- [Typechecking Django and DRF](https://sobolevn.me/2019/08/typechecking-django-and-drf) guide
+- [Testing mypy stubs, plugins, and types](https://sobolevn.me/2019/08/testing-mypy-types) guide
+
+It is also recommended to take a look at these resources:
+
+- [Awesome Python Typing](https://github.com/typeddjango/awesome-python-typing)
+
+
+## Dev documentation
+
+TODO
+
+
+## Dependencies
+
+We use `pip` to manage the dependencies.
+
+To install them you would need to activate your `virtualenv` and run `install` command:
+
+```bash
+pip install -r ./dev-requirements.txt
+```
+
+
+## Tests and linters
+
+We use `mypy`, `pytest`, `flake8`, and `black` for quality control.
+Here's [how we run our CI](https://github.com/typeddjango/django-stubs/blob/master/.travis.yml).
+
+### Typechecking
+
+To run typechecking use:
+
+```bash
+mypy ./mypy_django_plugin
+```
+
+### Testing
+
+There are unit tests and type-related tests.
+
+To run unit tests:
+
+```bash
+pytest
+```
+
+Type-related tests ensure that different Django versions do work correctly.
+To run type-related tests:
+
+```bash
+python ./scripts/typecheck_tests.py --django_version=2.2
+python ./scripts/typecheck_tests.py --django_version=3.0
+```
+
+Currently we only support two Django versions.
+
+### Linting
+
+To run auto-formatting:
+
+```bash
+isort -rc .
+black django-stubs/
+```
+
+To run linting:
+
+```bash
+flake8
+flake8 --config flake8-pyi.ini
+```
+
+
+## Submitting your code
+
+We use [trunk based](https://trunkbaseddevelopment.com/)
+development (we also sometimes call it `wemake-git-flow`).
+
+What the point of this method?
+
+1. We use protected `master` branch,
+   so the only way to push your code is via pull request
+2. We use issue branches: to implement a new feature or to fix a bug
+   create a new branch named `issue-$TASKNUMBER`
+3. Then create a pull request to `master` branch
+4. We use `git tag`s to make releases, so we can track what has changed
+   since the latest release
+
+So, this way we achieve an easy and scalable development process
+which frees us from merging hell and long-living branches.
+
+In this method, the latest version of the app is always in the `master` branch.
+
+
+## Other help
+
+You can contribute by spreading a word about this library.
+It would also be a huge contribution to write
+a short article on how you are using this project.
+You can also share your best practices with us.