Add simple contribution guide

CONTRIBUTING.md

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+# Contributing
+
+We love pull requests from everyone. Here are some instructions for
+contributing code to Open Food Network.
+
+Fork, then clone the repo:
+
+    git clone git@github.com:your-username/openfoodnetwork.git
+
+Follow the instructions in README.markdown to set up your machine.
+
+Make sure the tests pass:
+
+    rspec spec
+
+Make your change. Add tests for your change. Make the tests pass:
+
+    rspec spec
+
+Push to your fork and [submit a pull request][pr].
+
+[pr]: https://github.com/openfoodfoundation/openfoodnetwork/compare/
+
+At this point you're waiting on us. We may suggest some changes or
+improvements or alternatives.
+
+To increase the chance that your pull request is swiftly accepted:
+
+* Write tests
+* Use a style consistent with the rest of the codebase
+* Before submitting, [rebase your work][rebase] on the current master branch
+
+[rebase]: https://www.atlassian.com/git/tutorials/merging-vs-rebasing/workflow-walkthrough