This change adds a little documentation to the tests under
tools/testing/selftests/, based on akpm's explanation.

[akpm@linux-foundation.org: move from Documentation to tools/testing/selftests/README.txt]
Signed-off-by: NJeremy Kerr <jk@ozlabs.org>
Cc: Dave Young <dyoung@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>