Could not start application apns: exited in: APNS.start(:normal, [])

It seems that the documentation is incomplete.

I tried the config with my updated infos in the README example without success it seems that the apns config needs a pools section. Can you update the README.md ?

```
** (Mix) Could not start application apns: exited in: APNS.start(:normal, [])
** (EXIT) an exception was raised:
** (Protocol.UndefinedError) protocol Enumerable not implemented for nil
(elixir) lib/enum.ex:1: Enumerable.impl_for!/1
(elixir) lib/enum.ex:116: Enumerable.reduce/3
(elixir) lib/enum.ex:1636: Enum.reduce/3
(elixir) lib/enum.ex:1188: Enum.map/2
(apns) lib/apns.ex:35: APNS.start/2
(kernel) application_master.erl:273: :application_master.start_it_old/4
```