added CONTRIBUTING.md

CONTRIBUTING.md

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+Thanks for contributing to Growstuff!  We have different contribution
+guidelines depending on whether your change is a small one (a one-line
+bugfix or similar) or a larger one.
+
+## Small changes (no more than a couple of lines)
+
+Send us a pull request!  We will get one of our pairs of coders to
+review it.
+
+If you are interested in becoming a more regular contributor or working
+on larger features, please see the [Growstuff
+wiki](http://wiki.growstuff.org/) for more information.
+
+## Larger changes
+
+Growstuff does pair programming (two coders working together) for all
+significant changes.  This means that if you submit a pull request and
+weren't working in a pair, we can't merge it into the project as-is.
+
+If you would like to work on any larger change, we would appreciate it
+if you would get in touch with us first, preferably via our [mailing
+list](http://lists.growstuff.org/mailman/listinfo/discuss), and talk to
+us about it first.  We'll try and hook you up with a partner so you can
+work as a pair, either in person or remotely depending on where you are.
+The [Growstuff wiki](http://wiki.growstuff.org/) has lots more
+information on our dev process, if you're interested.
+
+If you submit a significant change without working in a pair, we will
+treat your work as an experimental "spike" and get one of our pairs of
+programmers to look over it and maybe use what you've done as the basis
+for re-implementing it using our processes.  **We'd much rather work with
+you, so please talk to us first!**
+