docs(contributing): add guidelines

CONTRIBUTING.md

@@ -0,0 +1,9 @@
+### Contributing
+
+When adding a new feature, please add an associated test, we want to make sure the project is full-covered.
+Do your changes on a dedicated branch, like `feature/facebook-auth` or `fix/bangular-refresh`, and when you're done, make your PR on `develop`.
+
+Please follow the AngularJS commit messages [conventions](https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit). It help us to generate the changelog.
+Squash your changes into one commit, it's easier to review.
+
+Thanks for your contribution! :cat: