Write up Developer Instructions (CONTRIBUTING.md)

[grahammcculloch]

Needs to include the following:

• Dev environment setup
• Code update procedure (issue -> branch -> PR -> merge)
• Deployment procedure
  • App Center (Android and iOS)
  • Code Push updates (staging and production, Android and iOS)
  • Native app updates (versioning, building)
    • Google Play Developer Console
    • App Store Connect