[webauthn] address needs of various webauthn spec audiences

equalsJeffH has just created a new issue for https://github.com/w3c/webauthn:

== address needs of various webauthn spec audiences ==
The webauthn spec is written for multiple audiences, including _at least_: 
* Relying Party deployers
* Web browser implementers
* OS platform implementers
* Authenticator vendors

PR #375 attempts to address this by adding a "spec roadmap" section to the introduction. this may or may not be sufficient -- we may want to consider creating "explainer" docs for one (eg RP deployers) or more audiences.


Please view or discuss this issue at https://github.com/w3c/webauthn/issues/585 using your GitHub account