Amend instructions

[grosch]

In the installation instructions please remind people to add Submissions to their app target dependencies. Took me a bit of futzing before I remembered I needed to do that.

[siemensikkema]

Agreed! I'll do that right away.

[siemensikkema]

@grosch I also forgot to mention that you need to register a provider in configure.swift:

try services.register(SubmissionsProvider())

I've added this to the documentation but I found something that doesn't quite make sense and I want to fix it and see what the impact on the documentation before I push.

[siemensikkema]

I updated the docs now.