Update documentation to add further information about using Thunderbird with git-imap-send.

Signed-off-by: Jeremy White <jwhite@codeweavers.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -376,9 +376,36 @@ Thunderbird
 
 (A Large Angry SCM)
 
+By default, Thunderbird will both wrap emails as well as flag them as
+being 'format=flowed', both of which will make the resulting email unusable
+by git.
+
 Here are some hints on how to successfully submit patches inline using
 Thunderbird.
 
+There are two different approaches.  One approach is to configure
+Thunderbird to not mangle patches.  The second approach is to use
+an external editor to keep Thunderbird from mangling the patches.
+
+Approach #1 (configuration):
+
+This recipe is current as of Thunderbird 2.0.0.19.  Three steps:
+  1.  Configure your mail server composition as plain text
+      Edit...Account Settings...Composition & Addressing,
+        uncheck 'Compose Messages in HTML'.
+  2.  Configure your general composition window to not wrap
+      Edit..Preferences..Composition, wrap plain text messages at 0
+  3.  Disable the use of format=flowed
+      Edit..Preferences..Advanced..Config Editor.  Search for:
+        mailnews.send_plaintext_flowed
+      toggle it to make sure it is set to 'false'.
+
+After that is done, you should be able to compose email as you
+otherwise would (cut + paste, git-format-patch | git-imap-send, etc),
+and the patches should not be mangled.
+
+Approach #2 (external editor):
+
 This recipe appears to work with the current [*1*] Thunderbird from Suse.
 
 The following Thunderbird extensions are needed:

Documentation/git-imap-send.txt

@@ -98,6 +98,20 @@ Using direct mode with SSL:
 ..........................
 
 
+CAUTION
+-------
+It is still your responsibility to make sure that the email message
+sent by your email program meets the standards of your project.
+Many projects do not like patches to be attached.  Some mail
+agents will transform patches (e.g. wrap lines, send them as
+format=flowed) in ways that make them fail.  You will get angry
+flames ridiculing you if you don't check this.
+
+Thunderbird in particular is known to be problematic.  Thunderbird
+users may wish to visit this web page for more information:
+  http://kb.mozillazine.org/Plain_text_e-mail_-_Thunderbird#Completely_plain_email
+
+
 BUGS
 ----
 Doesn't handle lines starting with "From " in the message body.