add Contributing

Contributing.md

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+# Contributing #
+## Branches ##
+Course consist from 2 branches 
+
+- master - for questions
+- answers - example answers implementations
+
+## PRs ##
+PRs are very welcome. They are a bit tricky though as project development goes in 2 branches.
+
+In case if you want to extend the course please commit to answers branch. 
+Then merge it to master and wipe out questions. 
+After that create 2 PRs - one for master and one for answers.
+
+