Documentation: Explain git-mergetool's use of temporary files

'git mergetool' creates '*.orig' backup files in its
default configuration.  Mention this in its documentation.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-mergetool.txt

@@ -72,6 +72,16 @@ success of the resolution after the custom tool has exited.
 	This is the default behaviour; the option is provided to
 	override any configuration settings.
 
+TEMPORARY FILES
+---------------
+`git mergetool` creates `*.orig` backup files while resolving merges.
+These are safe to remove once a file has been merged and its
+`git mergetool` session has completed.
+
+Setting the `mergetool.keepBackup` configuration variable to `false`
+causes `git mergetool` to automatically remove the backup as files
+are successfully merged.
+
 Author
 ------
 Written by Theodore Y Ts'o <tytso@mit.edu>