Added first draft of Scaling Strategies under an Advanced Topics heading

This is a FAQ on the mailing list, and I think Sensu is mature enough to have this documented.

This is just a first draft with my personal input obviously. I hope it can be refined by other experienced practitioners. 

I figure a first draft at this documentation is better than None.

Also, I'm starting a new Advanced Topics section. I figured more things would go here that are out of scope of the basic docs. Things like advanced handlers, custom filtering, remediation, etc)