add contributing guide

[ci skip]

CONTRIBUTING.md

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+### Please read before contributing
+
+1) If you have any questions about CanCan, search the [Wiki](https://github.com/ryanb/cancan/wiki) or use [Stack Overflow](http://stackoverflow.com/questions/tagged/cancan). Do not post questions here.
+
+2) If you find a security bug, **DO NOT** submit an issue here. Please send an e-mail to [ryan@railscasts.com](mailto:ryan@railscasts.com) instead.
+
+3) Do a small search on the issues tracker before submitting your issue to see if it was already reported / fixed. In case it was not, create your report including Rails and CanCan versions. If you are getting exceptions, please include the full backtrace.
+
+That's it! The more information you give, the more easy it becomes for us to track it down and fix it. Ideal scenario would be adding the issue to CanCan test suite or to a sample application.
+
+Thanks!