Add instructions for installing postfix.
Tweak target operating systems info

Prefer a 64-bit version of all Operating Systems
Update API docs to reflect new state update methods
Merge branch 'gh-pages-1418-update-api-docs' into gh-pages
changes to document statuses with @crowbot
Merge branch 'gh-pages-os-version' into gh-pages
Add documentation on installing with Vagrant.
Add next steps section.
Merge branch 'gh-pages-vagrant' into gh-pages
Merge branch 'gh-pages' of ssh://git.mysociety.org/data/git/public/alaveteli into gh-pages
Get rid of ALAVETELI_USER and ALAVETELI_HOME

It's not clear whether they are actual variables (they aren't).
Fix Postfix configuration instructions.

Copy the file changes used by install_site.sh
Fix a couple of typos
Add recipient groups
Add instructions for bounce message handling
Bit more info on what you should see on the happy path.
Reference Postfix as well as Exim - it's easier to setup.
Configure postfix non-interactively
And add the hostname to the domains to be delivered locally.
Don't focus on Ubuntu

Where Ubuntu and Debian differ, give both details.