Rewrite contributing guide (and switch to ReST)

CONTRIBUTING.rst

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+Thank you for your interest in contributing to Junction. We welcome all
+contributions and greatly appreciate your effort!
+
+Bugs and Features
+-----------------
+
+If you have found any bugs or would like to request a new feature, please do
+check if there is an existing issue already filed for the same, in the
+project's GitHub `issue tracker`_. If not, please file a new issue.
+
+If you want to help out by fixing bugs, choose an issue from the `issue
+tracker`_ to work on and claim it by posting a comment saying "I would like to
+work on this.". Feel free to ask any doubts in the issue thread.
+
+Once you have implemented the feature to an extent, go ahead and file a pull
+request, following the tips below. File a pull request early to get feedback as
+early as possible.
+
+Pull Requests
+-------------
+
+Pull Requests should be small to facilitate easier review. Keep them
+self-contained, and limited in scope. Studies have shown that review quality
+falls off as patch size grows. Sometimes this will result in many small PRs to
+land a single large feature.
+
+Checklist:
+
+1. All pull requests *must* be made against the ``master`` branch.
+2. Include tests for any functionality you implement. Any contributions helping
+   improve existing tests is welcome.
+3. Update documentation as necessary and provide documentation for any new
+   functionality.
+4. In case of UI changes, please include screenshots.
+5. Add yourself to ``CONTRIBUTORS.txt`` if you're not there already. :)
+
+If you do make any changes to models (modification or addition), make sure to
+run ``python manage.py makemigrations`` to enable the server to migrate existing
+data to the new models.
+
+Code Convention
+---------------
+
+We follow the `Django Coding Style`_ and enforce it using `flake8`_.
+
+In general, if flake8 is happy with your code, you should be fine. To use
+``flake8`` to check your code, you can use the following command::
+
+   $ flake8 --max-complexity=24 --statistics --benchmark --ignore=E5,F4 <project_dir>/
+
+.. _`issue tracker`: https://github.com/pythonindia/junction/issues
+.. _`flake8`: https://flake8.readthedocs.org/en/latest/
+.. _`Django Coding Style`: https://docs.djangoproject.com/en/2.2/internals/contributing/writing-code/coding-style/