Add plugin interface documentation

This commit starts the docs section on creating and using tempest
plugins. It describes how to create a plugin class, an entrypoint, and
a recommended dir structure. As well notes how to use tempest and
tempest-lib in creating a plugin. It'll likely be expanded in the
future as the plugin interface evolves.

Partially Implements bp external-plugin-interface

Change-Id: If2e0c47d0a719d869a3301ddbca07495c81cc498
diff --git a/doc/source/index.rst b/doc/source/index.rst
index f925018..44bfc88 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -10,6 +10,7 @@
    overview
    HACKING
    REVIEWING
+   plugin
 
 ------------
 Field Guides