Document coding standards and conventions

[japgolly]

Purpose:

• Consistency
• Keep current and new maintainers on the same (explicit) page
• Provide a doc for new contributors to read and adhere to
• Avoid needless back-and-forth between maintainers and contributors

Content:

• package structure
• file organisation (#476)
• browser support (#514)
• non-facades (eg. implicit ops or conversions like DOMNodeList => SeqLike)
  • location
  • naming conventions
  • exports (eg. @inline def css = raw.css)
• facades
  • def vs val
  • null handling (i.e. just document that we don't put it in types atm, hey maybe annotations would be a good idea?)
  • when/where/how to use js.Any
• PRs
  • prePR
  • changelog? or is release-drafter enough? (probably right?)