Facelift the guide

[pirj]

The guide has been open-sourced. Before doing any announcements, a couple of minor but important changes are needed:

• give Toptal proper credit for sponsoring all initial work on the guide:
  • gathering good and bad practices from existing code
  • researching and getting to the bottom of problems of improper use
  • extracting good patterns from the migration to Sidekiq
  • colleague's work on reviewing and proofreading the guide
  • fix TOC appearance on Github
  • make it clear that most but not all of the guidelines apply to everyone

[pirj]

@conf Thanks! 🙌