pagemap: add documentation for pagemap

Just a quick explanation of the pagemap interface from a userspace point
of view, and an example of how to use it (in English, not code).

Signed-off-by: Thomas Tuttle <ttuttle@google.com>
Cc: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>