Documentation/email-clients.txt: update Thunderbird docs with wordwrap plugin

Show how to install the "toggle wordwrap" extension in thunderbird.

Signed-off-by: Rob Landley <rlandley@parallels.com>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>