Add CONTRIBUTING guide

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Changes since v2.2:
 
+- Add CONTRIBUTING guide
 - Add Issue and Pull Request templates
 - Add Dockerfile
 - Fix fsnotify import

CONTRIBUTING.md

+# Contributing
+
+To develop on this project, please fork the repo and clone into your `$GOPATH`.
+
+Dependencies are **not** checked in so please download those separately.
+Download the dependencies using [`dep`](https://github.com/golang/dep).
+
+```bash
+cd $GOPATH/src/github.com # Create this directory if it doesn't exist
+git clone git@github.com:<YOUR_FORK>/oauth2_proxy pusher/oauth2_proxy
+dep ensure # Installs dependencies to vendor folder.
+```
+
+## Pull Requests and Issues
+
+We track bugs and issues using Github.
+
+If you find a bug, please open an Issue.
+
+If you want to fix a bug, please fork, create a feature branch, fix the bug and
+open a PR back to this repo.
+Please mention the open bug issue number within your PR if applicable.

README.md

@@ -420,3 +420,7 @@ server {
   }
 }
 ```
+
+## Contributing
+
+Please see our [Contributing](CONTRIBUTING.md) guidelines.