Add information about forking to contributing docs

[lyallstewart]

Purpose

Will add information to potential collaborators on how to collaborate via a fork, since users who are not part of our org (or added as an outside contribute) do not have write access, so cannot create branches/push code/PRs etc.

Is your feature request related to a problem? Please describe.

Our current contributes guide only applies to users who already have write level access to the repo, which does not include any outside contributions. We would have to add a section explaining forks that users from outside the staff team can use.

This could either be added to the current CONTRIBUTORS.md or it could be added as a wiki page.

Acceptance Criteria

Please list the requirements the implementation of this feature should meet.

• Contribution docs should have information on working off a fork