Add onboarding in README

[y9san9]

This would be nice enhancement to have onboarding explaining how the bot code designed. First step is to add readmes to all modules and then describe structure in ONBOARDING.md.

• [ ] main module docs and project structure
• [ ] bot module docs
  • [ ] telegram module docs
• [x] buildSrc module docs
• [x] db-migrations module docs
• [x] fsm module docs
• [x] random module docs
  • [x] jsonRPC module docs
• [x] utils module docs