Initial doc for tips on scaling Pulp

This initial doc covers an overview of Pulp's components (including diagram),
some information on scaling the httpd and worker tiers, and some basic
information on alerting and monitoring. Links are provided for some information
about Mongo, qpid/rabbit and httpd.

I have little knowledge of enterprise storage so I left that part commented out
to fill in at a later date.
Merge pull request #1180 from beav/scaling

Initial doc for tips on scaling Pulp