Initial kselftests README

1. Description of the key files in kernel's source tree related to
   running selftests:
   - `tools/testing/selftests/Makefile'
   - `tools/testing/selftests/lib.mk'
   - `tools/testing/selftests/run_kselftest.sh'
   - `tools/testing/selftests/kselftest/runner.sh'
   - `tools/testing/selftests/kselftest-list.txt'
2. Description of how the tests are organized and how does this abstract
   organization map to the files and directories in the
   `tools/testing/selftests/' subdir.
3. Practical tips for specific use cases
   - Printing the list of all available tests.
   - Running a specific test (instead of the whole collection).
   - Changing the test's timeout.

Author: pvts-mat