Documentation update
- make it clear that you have to have an existing repo to clone
- don't try to put the same key in different directories
- explain the .mercurial-server file.
usr/share/mercurial-server
usr/share/mercurial-server/init
usr/share/mercurial-server/mercurialserver
usr/share/doc/mercurial-server
etc/mercurial-server
etc/mercurial-server/remote-hgrc.d
etc/mercurial-server/keys
etc/mercurial-server/keys/root
etc/mercurial-server/keys/users