Add table of contents in README

Before submitting the PR make sure the following are checked:

[x] Feature branch is up-to-date with master (if not - rebase it).
[x] Squashed related commits together.
[-] Added tests.
[-] Updated documentation.
[-] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
[x] The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

rubocop / rubocop-rspec