Add contributing guidelines

CONTRIBUTING.md

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+# Contributing to Write Freely
+
+Welcome! We're glad you're interested in contributing to the Write Freely project.
+
+To start, we'd suggest checking out [our Phabricator board](https://phabricator.write.as/tag/write_freely/) to see where the project is at and where it's going. You can also [join the Write Freely forums](https://discuss.write.as/c/writefreely) to start talking about what you'd like to do or see.
+
+## Contributing code
+
+We gladly welcome development help, regardless of coding experience. We can also use help [translating the app](https://poeditor.com/join/project/TIZ6HFRFdE) and documenting it!
+
+**Before writing or submitting any code**, please sign our [contributor's agreement](https://phabricator.write.as/L1) so we can accept your contributions. It is substantially similar to the _Apache Individual Contributor License Agreement_. If you'd like to know about the rationale behind this requirement, you can [read more about that here](https://phabricator.write.as/w/writefreely/cla/).
+
+Once you've done that, please feel free to [submit a pull request](https://github.com/writeas/writefreely/pulls) for any small improvements. For larger projects, please [join our development discussions](https://discuss.write.as/c/writefreely) or [get in touch](https://write.as/contact) so we can talk about what you'd like to work on.