Fix doc example to install WP_Mock via composer

[unfulvio-godaddy]

Summary

Fixes instruction for installing WP_Mock package via composer, along with adding doc and readme improvements.

Closes: #213

Details

Contributor checklist

• [x] I agree to follow this project's Code of Conduct.
• [x] I have updated the documentation accordingly
• [x] I have added tests to cover changes introduced by this pull request
• [x] All new and existing tests pass

Testing

• [ ] Consult updated README.md file and affected doc pages

Reviewer checklist

- [ ] Code changes review - [ ] Documentation changes review - [ ] Unit tests pass - [ ] Static analysis passes

[coveralls]

coverage: 43.396%. remained the same when pulling fdfdad962719be1339cc3e5b5b2b312d6aa6bfcc on chore/fix-documentation-instructions into 8961bdade96a4c1dda1274a6cdd187913663f797 on trunk.

[unfulvio-godaddy]

@jeffpaul this wasn't added to a milestone because documentation isn't part of a release, and changes are/were instant, so it didn't belong to a release

[jeffpaul]

@unfulvio-godaddy we still tend to include PRs merged during a release cycle but not typically part of the release software in the same milestone. The reasoning here being if we're trying to triage things that happened around that timeframe we have good context of what happened then.