Explain how to add a missing repo

[lourot]

If the user feels like a repo is missing, they need to know what to do:

• maybe the repo has no star at all (we skip these repos), so they need to just give it a star.
• maybe we failed at discovering it, so we should implement a way for the user to add that repo to their user-settings (see #12)

This documentation could be accessed with a small link "Is there a missing repo?" at the bottom of each profile.