Added the relevant section in INSTALL about queues and daemons to get
the plugin runnig.
Made resource required, as otherwise XMPPHP will send invalid from JIDs
in it's stanzas. For example when my configuration didn't had the
resource part, outbound stanzas looked like this:
<message
from="gnusocial@example.de/"
to="flow@example.de"
type='chat'>
<body>
User "flow" on GNU Social has said that your
XMPP/Jabber/GTalk screenname belongs to them.
…
</body>
</message>
Note the '/' at the end of the from attribute, without an actual
XMPP resource. But according to RFC6122 2.1 "every allowable portion of
a JID MUST NOT be zero bytes in length". Causing a jid-malformed
response from the server.
Also, it's nice to know that debug=true will print out all sent and
received stanzas, which helped me to debug the problem.
Furthermore I add a note that if the XMPP services uses DNS SRV records,
'host' has to be configured (in cases where service host != xmpp domain).
