Documentation/email-clients.txt: add info about Claws Mail

Claws Mail GUI client works.
The client is available at http://www.claws-mail.org

Signed-off-by: Richard Leitner <dev@g0hl1n.net>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/email-clients.txt

@@ -76,6 +76,17 @@ When composing the message, the cursor should be placed where the patch
 should appear, and then pressing CTRL-R let you specify the patch file
 to insert into the message.
 
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Claws Mail (GUI)
+
+Works. Some people use this successfully for patches.
+
+To insert a patch use Message->Insert File (CTRL+i) or an external editor.
+
+If the inserted patch has to be edited in the Claws composition window
+"Auto wrapping" in Configuration->Preferences->Compose->Wrapping should be
+disabled.
+
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Evolution (GUI)