Add an internals doc to the documentation

Sometimes having narrative text describing how objects hang together it
handy for knowing what to do.

Change-Id: Ib12af2d993740d82b4f43de74b11ecefd1cd363a
diff --git a/doc/source/index.rst b/doc/source/index.rst
index 3c793da..8c12138 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -24,6 +24,7 @@
    launchers
    statsd
    client
+   internals
 
 Indices and tables
 ==================