v2, updated based on comments from Joe and Paul.

Update to Documentation/dontdiff, based on many updates to
various .gitignore patches over the last 2 years.

Initially begun by Michael Prokop <mika@grml.org>, with lots of
changes by Randy Dunlap.
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Michael Prokop <mika@grml.org>
Cc: Joe Perches <joe@perches.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>