Broken link to edit docs, perhaps should link to repo instead of directory to avoid having a branch in the URL. Also added blurb to clarify what branch doc edits should occur in.
The guidelines for contributing code were scattered across a section of contributing.md and collaboration-on-git.md. I've updated this to have separate contributing/code.md page with all the content in a single cohesive page. We also have contributing/documentation.md, contributing/issues.md and contributing/translation.md.
