Add documentation for `device.yml` specification

[bruno-f-cruz]

This is a placeholder issue.

Some important points to include:

• General device yml structure
• interfaceType documentation (Define a string interfaceType and establish documentation for well-known types #118 )
• device yml as a interface specification/documentation NOT a device specification file
• We should add a URL reference to the WhoAmI json/yml database to allow submissions to point to a known yml file

[PathogenDavid]

This indirectly ties into #118 since it needs somewhere for well-known type documentation to live.