Add contributing file

CONTRIBUTING.md

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+## Security issues
+
+If you have found a security related issue, please do not file an issue on
+GitHub or send a PR addressing the issue. Contact
+[Jonas](mailto:jonas.nicklas@gmail.com) directly. You will be given public
+credit for your disclosure.
+
+## Reporting issues
+
+Please try to answer the following questions in your bug report:
+
+- What did you do?
+- What did you expect to happen?
+- What happened instead?
+
+Make sure to include as much relevant information as possible. Ruby version,
+Pundit version, OS version and any stack traces you have are very valuable.
+
+## Pull Requests
+
+- **Add tests!** Your patch won't be accepted if it doesn't have tests.
+
+- **Document any change in behaviour**. Make sure the README and any other
+  relevant documentation are kept up-to-date.
+
+- **Create topic branches**. Please don't ask us to pull from your master branch.
+
+- **One pull request per feature**. If you want to do more than one thing, send
+  multiple pull requests.
+
+- **Send coherent history**. Make sure each individual commit in your pull
+  request is meaningful. If you had to make multiple intermediate commits while
+  developing, please squash them before sending them to us.