ssh: add documentation of SSH support

Based on work by Ilya Beda <ir4y.ix@gmail.com> on
https://bitbucket.org/ir4y/rhodecode/commits/branch/ssh_server_support , also
heavily modified by Mads Kiilerich and Thomas De Schampheleire.

Text related to the case where the parent directory of 'authorized_keys'
does not exist or is not writable, was contributed by Bradley M. Kuhn
<bkuhn@ebb.org>.