Published documentation

[viralpraxis]

Since rubocop-thread_safety is a part of rubocop organization it seems weird our docs are not listed in rubocop's official documentation site (see rubocop-minitest or rubocop-rspec for example). We are also not listed in "projects" dropdown list.

I've not yet figured out what exactly should be done to host our docs on docs.rubocop.org, but here are some useful resources:

1. https://docs.rubocop.org/rubocop/contributing.html#documentation
2. Documentation rubocop-rspec#972 (comment)

@koic Could you please clarify if we can fix this issue?

FYI @mikegee