README: prefer "init -r" to "ssh ... bup init"

README: prefer "init -r"  to "ssh ... bup init"

Recommend 'bup init -r SERVERNAME:PATH' instead of 'ssh SERVENAME bup
init'.

Update the example of making a backup to a remote server in README.md.
Replace 'ssh SERVENAME bup init' with 'bup init -r SERVERNAME'. The
latter doesn't only initialize the remote repository, but also the local
one (if it doesn't exist).  Augment 'bup {init,save} -r SERVERNAME'
commands with the path specifier to show the ability to specify the
remote path.

Signed-off-by: Tadej Janež <tadej.j@nez.si>
Reviewed-by: Gabriel Filion <gabster@lelutin.ca>
[rlb@defaultvalue.org: shorten/adjust commit summary; adjust and change
 tense of commit message.]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>