Added docs for hooks

Patch from Sias Mey

fixes #71

offlineimap.conf

@@ -183,6 +183,19 @@ remoterepository = RemoteExample
 
 # quick = 10
 
+# You can specify a pre and post sync hook to execute a external command.
+# in this case a call to imapfilter to filter mail before the sync process
+# starts and a custom shell script after the sync completes.
+# The pre sync script has to complete before a sync to the account will
+# start. 
+
+# presynchook = imapfilter
+# postsynchook = notifysync.sh
+
+# You can also specify parameters to the commands
+# presynchook = imapfilter -c someotherconfig.lua
+
+
 [Repository LocalExample]
 
 # This is one of the two repositories that you'll work with given the