Document how to load extensions when testing plugins that rely on them

[jimmycuadra]

Currently you have to re-register an extension on the Lita::Registry for each test, e.g. https://github.com/jimmycuadra/lita-digitalocean/blob/ebbb5e9c5f7ebac11e2bbdeedb3c793536d5820f/spec/spec_helper.rb#L16 but this is not at all obvious.