Documentation/development-process: add staging & mmotm info

Update explanation of mmotm.
Add explanation of drivers/staging/.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>