Add contribution guide

CONTRIBUTING.md

+## Find an issue
+
+First, find an area to work on within the shell or one of it's related projects.
+This may be:
+
+- An existing issue which has been reported
+- A feature that is missing that you would like to develop
+- An issue you've discovered that you would like to fix
+
+Once you have found something that you want to work on, submit your intent to
+the issue board, either by creating an issue for an issue which does not exist,
+or commenting on an issue that you are working on it.
+
+## Test your code
+
+Before submitting a PR, ensure that you've run your tests locally and that they
+pass. This can be done by running the following two commands:
+
+```sh
+cargo +nightly test --lib && bash examples/run_examples/sh
+```
+
+## Format your code
+
+In addition, format your code before submitting a PR. This will require that
+you've installed the `rustfmt` Cargo component.
+
+```sh
+cargo +nightly fmt
+```
+
+Now you're ready to submit your work for review!
+
+## Chatroom
+
+Send an email to [info@redox-os.org](mailto:info@redox-os.org) to request invitation for joining
+the developer chatroom for Ion. Experience with Rust is not required for contributing to Ion. There
+are ways to contribute to Ion at all levels of experience, from writing scripts in Ion and reporting
+issues, to seeking mentorship on how to implement solutions for specific issues on the issue board.
+
+## Discussion
+
+In addition to the chatroom, there's a [thread in the Redox forums](https://discourse.redox-os.org/t/ion-shell-development-discussion/682)
+that can be used for discussions relating to Ion and Ion shell development. These are mostly served
+by the GitHub issue board, but general discussions can take place there instead.