Nested Terminal commands and a blockquote under their respective instructions.

Typo: Reponse -> Response

* Update README for types, better Error examples, and whitespace.
* Add some newlines.

Also cleaned up all OS version checks that are no longer necessary.

Add the `public` identifier to the `BackendError` enum in the part about using Generics for Serialization.

Reason:
The extension of Request throws an error ("Method cannot be declared public because its parameter uses an internal type") unless the enum is public.

Add the `public` identifier to the `BackendError` enum in the part about using Generics for Serialization.

Reason:
The extension of Request throws an error ("Method cannot be declared public because its parameter uses an internal type") unless the enum is public.

Update cocoapod install to be compatible with cocoapod 1.x as it now requires a concrete target.