Add some documentation

To-do list:

• Add infrastructure
• Fill in the introduction
• Add a code of conduct
• Add a contributing section
  • Mention how to report security issues
• Add a changelog for user-facing changes since Conduit 0.7.0

Future work:

• Document reverse proxy configuration
• Document Grapevine configuration
• Document database compatibility status and caveats
  • See also #17
  • I think this will probably take the form of a "Migrating" section with a "To Grapevine" and "Away from Grapevine" subsection.
• Document development workflows