Add a procedure to accpet external repository into subaru_pfs organization

There are several request to put external (e.g. collaborator personal) repository at github into subaru-pfs github organization. It is better to seek 'easy' way to perform it and add as a procedure in operation document.

For now, it seems we can take two options:

• transfer existing repository to one of organization admin, and admin will do all after that
• temporary assign some permission level to whom want to transfer repository, and ask them to do, and revert permission

Considering number of interactions and security, it would be better to take first option.