Bring back documentation on Silex

Commit 1d891f0cb7b didn't ship with the Silex documentation and it has
never been pulled in the repository so this commit bring it back.

The guide is based on the PropelServiceProvider repository's README
file which is up to date.
Move the insert part to the bottom of the build guide

In the buildtime guide, the insert process is explained before the
necessary stuff is set up so the user can't get the schema inserted in
the database.