Document the `have_http_status` matcher.

Outline the intended use cases for the `have_http_status` matcher:

- It is designed to work of "response" objects
- It should work with controller, request, and feature specs
- It allows checking against:
  - Specific numeric status codes
  - Any of the Rails provided symbolic names for status codes
  - General status code categories
Create `have_http_status` matcher.

The matchers passes if the provided actual `response`, or Capybara
`page`, has a matching HTTP status code. The following symbolic status
codes are allowed:

  - `Rack::Utils::SYMBOL_TO_STATUS_CODE`
  - One of the defined `ActionDispatch::TestResponse` aliases:
    - `:error`
    - `:missing`
    - `:redirect`
    - `:success`

The inspiration for this matcher comes directly from Rails' `assert_response`:

  https://github.com/rails/rails/blob/ca200378/actionpack/lib/action_dispatch/testing/assertions/response.rb#L22-L38

Thank you to all the Rails devs who did the heavy lifting on
`assert_response`!
Add changelog note.