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
• buildSrc module docs
• db-migrations module docs
• fsm module docs
• random module docs
  • jsonRPC module docs
• utils module docs