Documentation: Update kselftest.txt

Add document for how to install selftests.

Signed-off-by: Wang Long <long.wanglong@huawei.com>
Acked-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kselftest.txt

@@ -54,6 +54,22 @@ To run the hotplug tests:
 - note that some tests will require root privileges.
 
 
+Install selftests
+=================
+
+You can use kselftest_install.sh tool installs selftests in default
+location which is tools/testing/selftests/kselftest or an user specified
+location.
+
+To install selftests in default location:
+   $ cd tools/testing/selftests
+   $ ./kselftest_install.sh
+
+To install selftests in an user specified location:
+   $ cd tools/testing/selftests
+   $ ./kselftest_install.sh install_dir
+
+
 Contributing new tests
 ======================