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: 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>

tools/testing/selftests/README.txt

+Linux Kernel Selftests
+
+The kernel contains a set of "self tests" under the tools/testing/selftests/
+directory. These are intended to be small unit tests to exercise individual
+code paths in the kernel.
+
+Running the selftests
+=====================
+
+To build the tests:
+
+  $ make -C tools/testing/selftests
+
+
+To run the tests:
+
+  $ make -C tools/testing/selftests run_tests
+
+- note that some tests will require root privileges.
+
+
+To run only tests targetted for a single subsystem:
+
+  $  make -C tools/testing/selftests TARGETS=cpu-hotplug run_tests
+
+See the top-level tools/testing/selftests/Makefile for the list of all possible
+targets.
+
+
+Contributing new tests
+======================
+
+In general, the rules for for selftests are
+
+ * Do as much as you can if you're not root;
+
+ * Don't take too long;
+
+ * Don't break the build on any architecture, and
+
+ * Don't cause the top-level "make run_tests" to fail if your feature is
+   unconfigured.