Add Contributing document from wiki

CONTRIBUTING.md

@@ -0,0 +1,33 @@
+## How to contribute
+If you can write source code to implement missing features or fix bugs, changes
+can be submitted for review via github's pull requests system.
+
+If you aren't able to contibute source code, we are also happy to see improved
+documentation of the expected behaviour on sites like the gtamodding wiki.
+Improving this documentation makes it easier to implement the correct behaviour.
+
+## Submitting Pull Requests
+
+If you are submitting a pull request for OpenRW:
+
+1. Include a reference to any issue the pull request is intended to address.
+2. Submit unrelated changes in different pull requests.
+3. If you aren't ready to merge your changes but are looking for feedback, include "[not ready]" in the title.
+4. Changes in behaviour should be shown to match the behaviour in the original game, if applicable.
+5. Ensure that the test suite still passes after your change, updating tests if required.
+
+## Guidelines
+### What will be accepted
+* Changes that increase compatibility with the game data.
+* Fixes for technical issues, such as wide-screen or UHD resolution support.
+* Feature enhancements, such as improved support for different input schemes.
+
+### What will not be accepted
+* Changes where the ownership of the code is in question.
+* Changes to support non-GTA titles.
+
+## Evaluating pull requests
+
+When a pull request is submitted, at least two members of the organisation should have reviewed the changes before merging.
+
+