Add contribution guide

CONTRIBUTING.md

+### Contributing to this repo
+
+* Fork the repository
+* Clone your fork using `https://git.amrita.edu/<your_username>/hodor-webapp`
+* Make a new branch with your changes using `git branch -b <branch_name>`. For
+ instance, if you are working on adding tests, you should do something like `git branch -b add_tests`.
+* Send in a pull-request.
+
+More instructions coming soon :)
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