Add unit test section to the field guide index

The unit tests are another class of tests in tempest, but they were
excluded from the field guide index. This commit adds a section at the
bottom of the index for the unit test field guide.

Change-Id: I0f081c7a94bca46322381f8a5d28d7cadf334bbc
diff --git a/tempest/README.rst b/tempest/README.rst
index b840fba..fb25151 100644
--- a/tempest/README.rst
+++ b/tempest/README.rst
@@ -73,3 +73,11 @@
 completely legitimate for Tempest to include tests of 3rdparty APIs,
 but those should be kept separate from the normal OpenStack
 validation.
+
+:ref:`unit_tests_field_guide`
+-----------------------------
+
+Unit tests are the self checks for Tempest. They provide functional
+verification and regression checking for the internal components of tempest.
+They should be used to just verify that the individual pieces of tempest are
+working as expected.
diff --git a/tempest/tests/README.rst b/tempest/tests/README.rst
index cb592e5..e54d4c0 100644
--- a/tempest/tests/README.rst
+++ b/tempest/tests/README.rst
@@ -1,4 +1,4 @@
-.. _unit_tests.rst:
+.. _unit_tests_field_guide:
 
 Tempest Field Guide to Unit tests
 =================================