Update vagrant README to include build instructions

.gitignore

@@ -58,3 +58,4 @@ mydbstub.py*
 
 /unittests/lib_pubsub_provider_actual.py
 
+ubuntu-xenial-16.04-cloudimg-console.log

vagrant/README.rst

@@ -35,4 +35,29 @@ host:
 Building
 --------
 
-TBW...
+The Vagrant VMs are provisioned to contain everything they need to build
+Phoenix packages, except for two things: the source code and a build script to
+do all the work. To make this a little simpler, the VMs are also configured to
+share the ``{PhoenixRoot}/dist`` folder, and the
+``{PhoenixRoot}/vagrant/scrtpts`` folder so they can access those two things
+as well.
+
+To perform a build there must be a Phoenix source tarball located in the
+``{PhoenixRoot}/dist`` folder, which can either be generated with the
+``build.py dox etg sip sdist`` command, or one of the source tarballs from a
+release or the snapshots server can be used. With that file in place then a
+build for a specific distro can be done with just a few vagrant commands, for
+example::
+
+    cd vagrant/ubuntu-16.04-x64
+    vagrant up
+    vagrant ssh -c "scripts/build.sh ubuntu-16.04-x64"
+    vagrant halt
+
+Please note that currently this will do up to 6 complete builds, one for each
+supported version of Python on that distro, for both gtk2 and gtk3.  The wheel
+files generated by the build will be placed in subfolders of
+``{PhoenixRoot}/dist``.
+
+TODO: Add the ability to select a single Python and gtk option.
+