Improve manual installation instructions

Summary:
List extensions to install, provide instruction for ast.

Explain phpdox can't currently be added to Composer, as there are
some conflictual dependencies.

Test Plan:
A new installation has been done pn the Cloud9 platform, in a fresh
Ubuntu Trusty container + PHP 7.1 through ondrej/php PPA.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D844