Update install guide to point at install scripts

Summary:
Point users at the install scripts. Fix some common points of confusion and
update some other documentation points.

The links point to a directory which ultimately serves these scripts out of the
git checkout on the machine:

http://phabricator.com/rsrc/install/install_rhel-derivs.sh
http://phabricator.com/rsrc/install/install_ubuntu.sh

Test Plan:
Read documentation.

Reviewed By: aran
Reviewers: codeblock, jungejason, tuomaspelkonen, aran
Commenters: jungejason
CC: aran, epriestley, jungejason
Differential Revision: 439