Document coding setup / workflows for future contributors and our reference

[HEdingfield]

We should document our IntelliJ setup, which plugins we use, how to use Gradle in IntelliJ and via the CLI, which checkboxes should be used when committing, etc.

This popped up in #386 and #387 since it became apparent that we need instructions for enabling the Checkstyle-IDEA plugin in IntelliJ, and how to run checkstyle using Gradle, and the fact that the HTML reports are generated locally in rcv\build\reports\checkstyle.

This could be done by creating a CONTRIBUTING.md file, as mentioned in these GitHub docs. Or maybe another method is better (e.g. keeping an issue open / for reference, like how we've been using #289).