Add tempest config guide doc

This commit adds a new doc page to the tempest documentation to
provide a guide to configuring tempest. It currently contains one
section to elaborate on the credentials and auth options in the config
file. This doc will be expanded in the future to add details around
other pieces of tempest configuration.

Change-Id: Iaf7cefc6a5e508437e4b4620ba5bfaa221f3611b
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 1f06bc5..cb1c504 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -29,6 +29,15 @@
    field_guide/thirdparty
    field_guide/unit_tests
 
+---------------------------
+Tempest Configuration Guide
+---------------------------
+
+.. toctree::
+   :maxdepth: 2
+
+   configuration
+
 ---------------------
 Command Documentation
 ---------------------