Update CONTRIBUTING.md to include PR process / forking information

[adammcarter]

Motivation

Hi all, when making my first contribution to Swift Syntax (🎉) it wasn't quite clear on the process to create PRs within this repo - see the comment from here: swiftlang/swift-syntax#1936 (comment) onwards in that thread.

Proposed solution

From that conversation it was suggested to open an issue to update the CONTRIBUTING.md both in Swift Syntax and here in the Swift repo to include the steps for non-maintainers to open their very own PRs.

The information on this post: swiftlang/swift-syntax#1936 (comment) from @Matejkob was super useful and would be good enough to copy and paste in to the CONTRIBUTING.md for any future travellers too IMO.

Thanks in advance!

Alternatives considered

No response

Additional information

No response

[adammcarter]

For the sake of alignment and any stepping on toes, it's likely this PR won't be implemented by me until the Swift Syntax equivalent is done where presumably both will have the same content.

The WIP PR for Swift Syntax is here: swiftlang/swift-syntax#2577