Include RSpec 3 changes for inferred metadata.

- Explain the move away from inferred metadata types
- State that default configurations will add the new setting to include
  inferred metadata by default due to all of the online tutorials
- List common Rails metadata `:type` values
- Add tip about mirroring `app/` and `lib/` filesystem structure in
  `spec/`
Merge pull request #1041 from rspec/fix-directory-structure-doc

Include RSpec 3 changes for inferred metadata.