Add contributing guidelines

CONTRIBUTING.md

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+### Contributing A Patch
+
+If you're working on an existing issue, such as one of the `help-wanted` ones
+above, simply respond to the issue and express interest in working on it.  This
+helps other people know that the issue is active, and hopefully prevents
+duplicated efforts.
+
+If you want to work on a new idea of relatively small scope:
+
+1. Submit an issue describing your proposed change to the repo in question.
+1. The repo owners will respond to your issue promptly.
+1. If your proposed change is accepted, fork the repo, develop, and test your changes.
+1. Submit a pull request.
+
+If you want to work on a bigger idea, submit an issue describing your proposed design.
+
+### Adding dependencies
+
+If your patch depends on new packages, add that package with
+[`godep`](https://github.com/tools/godep).  Follow the [instructions to add a
+dependency](docs/devel/development.md#godep-and-dependency-management).
+