Get rid of the git-gerrit.username config variable

It's not really necessary because the user can inform its username via
git-gerrit.baseurl instead.

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>
Document the authentication procedure

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>
Improve documentation

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>