readme should be less verbose

[zpatrick]

Instead of going over each request/client option, I should have a slimmed down version in the readme and examples of each option in godoc, e.g.:

# Request Options
* [Header](#link-to-example) - Add a header to a request
* [Basic Auth](#link-to-example) - Add basic auth  to a request

# Client Options
* [Doer](#) - Specify which object performs the http.Request (the default is http.DefaultClient)

# Error Handling
## using rcilent.ResponseError
## using ResponseReader() option