Remove old docs folder.
Add https://github.com/unittest-cpp/unittest-cpp.wiki.git as submodule 'docs'.
Add Documentation, Pre-requisites, and Downloading instructions to main README.
Add link to contributor graph under 'Contributors'.
Incorporate new commits in wiki.
Incorporate latest wiki docs.
Merge pull request #41 from pjohnmeyer/documentationUpdates

Documentation updates from my repository. Further doc updating will be done directly on master.
Fix spelling error in README
Add 'empty' builds directory per the wiki. (The folder is not actually empty as it includes a .gitignore.)