Add Contributing resolves lyef#3

Author: willianjusten

CONTRIBUTING.md

@@ -0,0 +1,12 @@
+# Contributing
+
+1. Fork it!
+2. Create your feature branch: `git checkout -b my-new-feature`
+3. Commit your changes: `git commit -m 'Add some feature'`
+4. Push to the branch: `git push origin my-new-feature`
+
+*Remember, you only call pull if your tests and lint pass.*
+
+**After your pull request is merged**, you can safely delete your branch.
+
+### [<-- Back](https://github.com/afonsopacifer/react-pomodoro/)