Improve README setup instructions

[mweidner037]

Fixes #196

With thanks to @sriedel for the detailed feedback!

[mweidner037]

I removed the applications: config since that should no longer be necessary - mix should pick up our def applications() function and start the process automatically. Also, using applications (instead of extra_applications) will turn off the automatic behavior for all modules, which is probably not what you want. https://elixirforum.com/t/why-do-we-use-application-function-in-mix-exs/23834/2

[scottmessinger]

Looks great! I have one question about the Repo.pool/0 function

[scottmessinger]

@mweidner037 In our code base, we do %{pid: pid} = Ecto.Repo.Registry.lookup(__MODULE__). Should we switch to this form or are they equivalent?

[mweidner037]

They appear to be equivalent, though perhaps the Ecto.Adapter.lookup_meta form is preferable since that function is actually documented. I'll update it when changing the Mongo Ecto versions.