Add reminder that _spec files are run by default.

Closes #1115
Remove app upgrade notes in favor of linked docs.

The feature notes have all of the details of how to properly upgrade.
These are hosted online and were already linked in the above section.
Remove the duplication as it detracts from the remainder of the README.
Use top-level namespace for `describe`.

Add consistent whitespace between `require` and the first
`RSpec.describe`.
State spec type metadata and default directories.

The directories where specs reside now essentially defaults with RSpec
3. Update make this clear and state what equivalent metadata is required
to mirror the functionality in other specs.
Use the new `have_http_status` matcher.
Remove explicit anchor tags.

Github READMEs now automatically add anchor links to the headers.
Various formatting updates.

- Be consistent on how lists are defined by using a dash
- Add missing backticks around code words
- Update some descriptions with clearer information
- Add some missing hyper links
Fix header nesting to resided under the project.
Whitespace only update.
Note extract of `stub_model` and `mock_model`.
Merge pull request #1117 from rspec/readme-doc-updates

Readme doc updates