I had added a generic contributing guide awhile back with the intentions of re-writing it and adding our project-specific bits, but unfortunately I never really got around to this.

However, now that the HAL has been unified, we are using xtask automation, and the examples require metadata, I think it's important that we clearly document what's required and what's expected for contributors.

Slightly tangential, but we should also update our PR template to point users to this file and ensure they have read it first.