Gopass: Document how command `git-credential` works

[worldofpeace]

[veecue]

I'd suggest to add a subcommand that directly executes git config --global credential.helper '!gopass git-credential $@' (the command is already stated in the description (see gopass help git-credential) and maybe link the git credential manpage.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[veecue]

I'm on it, hope I'll finish it today

[worldofpeace]

Thanks @veecue :sparkle: