Add introduction to libra documentation

Includes diagram of what a Libra based LBaaS solution should look like

Change-Id: Ia5caec0a6b0d566a1a348dc90f9478df2fbfb4db

doc/index.rst

@@ -4,5 +4,6 @@ Load Balancer as a Service Device Tools
 .. toctree::
    :maxdepth: 3
 
+   introduction
    client/index
    worker/index

doc/introduction.rst

@@ -0,0 +1,29 @@
+Introduction
+============
+
+Libra is a Load Balancer as a Service (LBaaS) system originally designed by
+Hewlett-Packard Cloud Services.  It consists of three of the core components
+required to get LBaaS working:
+
+* A command line client
+* A node pool manager to keep a warm spare pool of load balancers ready
+* A node worker to asyncronusly communicate to the API server
+
+It does not (yet) include the API server itself or HAProxy.  The API server is
+based on the Atlas API system but communicates to the workers using gearman.
+
+Layout
+------
+
+.. image:: libralayout.png
+
+Here you can see that the pool manager spins up the required Nova nodes with
+the load balancer image.  It then hands the details of this node over the the
+API server.
+
+The client sends an Atlas API request to the API server which in-turn sends the
+configuration information to the worker on the load balancer node.  The worker
+has a plugin API to speak to multiple load balancer types but is currently
+designed to use HAProxy.
+
+The parts of this diagram in orange are provided by the libra codebase.