Issue w3c#1045 - Add Platform-Specific Implementation Guidance

Text from w3c#1045 (comment)

index.bs

@@ -405,6 +405,12 @@ A variety of additional use cases and configurations are also possible, includin
 - A [=[RP]=] prompts the user for their [=authorization gesture=] in order to authorize a single transaction, such as a payment
     or other financial transaction.
 
+##  Platform-Specific Implementation Guidance ## {#platform-impl-guidance}
+
+This specification defines how to use Web Authentication in the general case. When using Web
+Authentication in connection with specific platform support (e.g. apps), it is recommended to see
+platform-specific documentation and guides for additional guidance and limitations.
+
 # Conformance # {#conformance}
 
 This specification defines three conformance classes. Each of these classes is specified so that conforming members of the class