[documentation] - How can PL request admin access

Our readme.md is some missing information on how PL's can request admin access for their Github projects.

This was discussed and implemented here: https://github.com/EclipseFdn/eclipsefdn-github-sync/issues/41#issuecomment-698486172

Projects can request admin access for PL by opening a bug via Bugzilla > Github. They should explain with their request why they need Admin access.

Webmaster will review the request and make the required changes if approved.