Improve README

[JokerQyou]

Let's just put everything in the main README, instead of splitting them into subdirectories. It should include these sections:

• Quick start guide:
  • How to get a local instance running in 10 seconds?
  • How to add extensions?
• Production deployment:
  • How to add SSL termination via reverse proxy?
  • Upgrade guide: how to run migrations?
• Advanced topics & development & contribution:
  • How to build your own docker images?
  • How to use the official Ruby server instead of the Golang server? (Let's do this later)