new rsync plugin

doc/plugins/contrib/rsync.mdwn

@@ -0,0 +1,21 @@
+[[!template id=plugin name=rsync core=0 author="[[schmonz]]"]]
+
+[[!template id=gitbranch branch=schmonz author="[[schmonz]]"]]
+
+This plugin allows ikiwiki to push generated pages to another host
+by running a command such as `rsync`.
+
+### Usage
+7. Enable automated SSH key exchange between ikiwiki and the remote
+   host. [keychain](http://www.gentoo.org/proj/en/keychain/) makes
+   it easy to use a passphrase-protected key for this purpose. It's
+   also a good idea to specify the exact command line to be permitted
+   in the remote host's `$HOME/.ssh/authorized_keys`.
+7. Set `rsync_command` in your setup file. If you're using a
+   passphrase-protected key, then set `rsync_command` to a shell
+   script which reads `keychain`'s current state before calling
+   `rsync`.
+
+### Implementation details
+* The plugin relies on a new "postrefresh" hook called at the very end of
+  `IkiWiki/Render.pm:refresh()`.