IMAP keyword sync: add Maildir configuration example

Signed-off-by: Igor Almeida <igor.contato@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap.conf

@@ -536,6 +536,31 @@ localfolders = ~/Test
 #
 #filename_use_mail_timestamp = no
 
+# This option stands in the [Repository LocalExample] section.
+#
+# Map IMAP [user-defined] keywords to lowercase letters, similar to Dovecot's
+# format described in http://wiki2.dovecot.org/MailboxFormat/Maildir . This
+# option makes sense for the Maildir type, only.
+#
+# Configuration example:
+#      customflag_x = some_keyword
+#
+# With the configuration example above enabled, all IMAP messages that have
+# 'some_keyword' in their FLAGS field will have an 'x' in the flags part of the
+# maildir filename:
+#      1234567890.M20046P2137.mailserver,S=4542,W=4642:2,Sx
+#
+# Valid fields are customflag_[a-z], valid values are whatever the IMAP server
+# allows.
+#
+# Comparison in offlineimap is case-sensitive.
+#
+# This option is EXPERIMENTAL.
+#
+#customflag_a = some_keyword
+#customflag_b = $OtherKeyword
+#customflag_c = NonJunk
+#customflag_d = ToDo
 
 [Repository GmailLocalExample]