Developing apps: add some more documentation

[sohkai]

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Some "best practices" for both for the contracts and the frontends:

• Frontend
  • How to structure the webworker script and the data it returns
    • Expectations for handling events:
      • Caching assumptions
      • Idempotence
      • Extraneous and undecoded events
      • Always expected to return current state
  • Client APIs available
    • Responsive menu panel control
    • Local identities
  • UX patterns
    • IdentityBadges everywhere
    • Usage of toasts
    • Sidepanel for actions
  • Set up aragon.js for local development
  • aragonUI
    • Add guidance for usage of relative paths for assets when building apps #54
  • Frontend testing flows (for local and live networks)
• Contracts
  • Style guide: Add solidity style guide #118
  • Time mocks (and proper usage in tests)
  • Usage of mappings vs. arrays
  • Radspec strings (tone, imperative voice)
• Etc
  • Add Aragon One's recommended development flow #30

[0xGabi]

Give some context to the aragonAPI - Javascript Quick Start Guide.