The recommended way to extend or alter CORE's functionality is via plugins.
Documentation on other functionality is available as well but plugin
developers should not rely too heavily on it. The plugin API is meant to be
quite stable but the underlying functionality behind the API may be subject
Stores already running some version of Fannie may want to start with
the Porting documentation. It's more of a step by step approach that
makes sense when you already have a bunch of code as opposed to someone
who's starting from scratch.