Summary:
the README didn't have explicit instructions for ubuntu 16.04 LTS...  now it does.

i also added a section for how to handle building folly w/a custom boost installation.
Closes https://github.com/facebook/folly/pull/664

Reviewed By: eduardo-elizondo

Differential Revision: D5694143

Pulled By: yfeldblum

fbshipit-source-id: e2786d045c3459cac49c8b052d90ca24d10f7922