Add CONTRIBUTING.md

CONTRIBUTING.md

+# CONTRIBUTING
+
+## Issues
+
+The issues of this repository refer only to the development of the tool. Other
+resources are:
+
+- [CocoaPods/Specs](https://github.com/CocoaPods/Specs): Submit new Pods and
+  report broken ones.
+- [Mailing List](http://groups.google.com/group/cocoapods): Usage & general
+  questions.
+
+Properly written and formatted issues are more likely to receive the
+appropriate attention.  If you don't care... we don't care.
+
+##### Bug reports
+
+Please post all the relevant information including the version of CocoaPods and
+any template printed by the tool.
+
+If there is a regression in the projects generated by CocoaPods please include
+the output (redacted if needed) of one of the following commands:
+
+```
+$ xcodeproj target-diff
+$ xcodeproj project-diff
+```
+
+If you are familiar with Ruby, making a pull request with a failing test case
+can speed up the resolution of the bug. If the issue is more complex you can
+add an [integration test](https://github.com/CocoaPods/CocoaPods/tree/master/spec/integration).
+which doesn't require any ruby knowledge.
+
+
+##### Feature requests
+
+Please try to be precise about the proposed outcome of the feature and how it
+would related with the existing ones.
+
+## Pull Requests
+
+We love pull requests and if a contribution is significative we tend to offer
+push access.
+
+All the contributions are intended to be licenced under the MIT license by
+their respective authors.
+