Doc: update kselftest.txt with details on how to run tests after install

Update kselftest.txt with details on how to run tests after install.
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kselftest.txt

@@ -70,6 +70,17 @@ To install selftests in an user specified location:
    $ cd tools/testing/selftests
    $ ./kselftest_install.sh install_dir
 
+Running installed selftests
+===========================
+
+Kselftest install as well as the Kselftest tarball provide a script
+named "run_kselftest.sh" to run the tests.
+
+You can simply do the following to run the installed Kselftests. Please
+note some tests will require root privileges.
+
+cd kselftest
+./run_kselftest.sh
 
 Contributing new tests
 ======================