Add missing documentation for core functions.

- Ensure the Concerns module is required when requiring
‘rspec/sleeping_king_studios/all’.
- Update README for ActiveModel matchers.
Update README, docs for ActiveModel, BuiltIn matchers.
Update README, docs for core matchers.

Current spec status:
- 2.0: 427 examples, 0 failures in 0.10975 seconds.
- 2.1: 492 examples, 0 failures in 0.1092 seconds.
Merge pull request #9 from sleepingkingstudios/upkeep-documentation

Update documentation.