git-svn: add documentation for --add-author-from option.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-svn.txt

@@ -65,6 +65,12 @@ COMMANDS
 	When retrieving svn commits into git (as part of fetch, rebase, or
 	dcommit operations), look for the first From: or Signed-off-by: line
 	in the log message and use that as the author string.
+--add-author-from;;
+	When committing to svn from git (as part of commit or dcommit
+	operations), if the existing log message doesn't already have a
+	From: or Signed-off-by: line, append a From: line based on the
+	git commit's author string.  If you use this, then --use-log-author
+	will retrieve a valid author string for all commits.
 --username=<USER>;;
 	For transports that SVN handles authentication for (http,
 	https, and plain svn), specify the username.  For other