Update readme; fix typos, expands some sections; add screenshots of request in state

[pianostringquartet]

I encountered some issues while following the readme. This PR fixes some typos/issues as well as expands some parts.

register-events and register-all are 1-arity

Re-frame-request register-events and register-all appear to be 1-arity functions, whereas README.md currently shows them as requires options:

I suggest using (register-events {}) and (register-all {}) instead, if you have no opts.

cljs-ajax: :json instead of (json-response-format)

The README.md uses :response-format (json-response-format), but this throws an error:

I’ve found :response-format :json works instead.

(I’m using cljs-ajax/cljs-ajax {:mvn/version "0.8.1"}.)

unregistered event: :no-op

Instead of using [:no-op], I define a real event handler for the on-failure case.

Expanded explanation of what it means to 'track the request in state'

I've provided explanations of the success and failure cases, along with what that looks like in state, using re-frame-10x's UI.