Document that '%' needs encoding as '%%' in offlineimap.conf

We use python's SafeConfigParser, and this is a built in "feature" used
for interpolating variables. But it imples that '%' needs encoding as
'%%'.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

Changelog.rst

@@ -11,6 +11,7 @@ WIP (add new stuff for the next release)
 * bump bundled imaplib2 library 2.29 --> 2.33
 * Actually perform the SSL fingerprint check (reported by J. Cook)
 * Curses UI, don't use colors after we shut down curses already (C.Höger)
+* Document that '%' needs encoding as '%%' in *.conf
 
 OfflineIMAP v6.5.3.1 (2012-04-03)
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