Add tip sections to the usage page

Add a tip on how to use kunit_tool, and another tip on how to build the
kernel.

Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Change-Id: I03bf82013670f54cc88dfa34227f4cfe4ebf939d

usage/index.rst

diff --git a/usage/index.rst b/usage/index.rst
index 9a4aea7..e7ec80b 100644
--- a/usage/index.rst
+++ b/usage/index.rst
@@ -27,11 +27,44 @@
 
 Once you have the ``kunitconfig`` file, just run:
 
-``$ ./tools/testing/kunit/kunit.py run``
+.. code-block:: bash
+
+   ./tools/testing/kunit/kunit.py run
+
+.. tip::
+
+   You may want to run KUnit with flags like:
+
+   .. code-block:: bash
+
+      ./tools/testing/kunit/kunit.py run --timeout=30 --jobs=24 --defconfig
+
+   - ``--timeout`` sets a maximum amount of time to allow tests to run.
+   - ``--jobs`` sets the number of threads to use to build the kernel.
+   - ``--defconfig`` uses an default kunitconfig in the kernel source.
+
+   For more information on these and other flags, try running:
+
+   .. code-block:: bash
+
+      ./tools/testing/kunit/kunit.py run --help
 
 This will build a UML (User Mode Linux) kernel, run the specified tests, and
 print the results (nicely formatted) to the screen.
 
+.. tip::
+
+   If you get the error message: ``/bin/sh: flex: command not found`` or
+   similarly ``/bin/sh: bison: command not found``, you are most likely missing
+   the flex and bison packages. On a system using the apt package manager you
+   can install them with
+
+   .. code-block:: bash
+
+      sudo apt-get install flex bison
+
+   For more information on building the kernel, see
+   https://www.kernel.org/doc/html/latest/process/changes.html
 
 Writing tests
 =============