selftests: add a simple doc

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: Jeremy Kerr <jk@ozlabs.org>
Cc: Dave Young <dyoung@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>