Fix #551 - Define a Store method for PublicKeyCredential.

The Store operation isn't defined for PublicKeyCredential, even though it
is inherited from Credential Management. This defines that operation as
always resolving with an error.

index.bs

@@ -985,6 +985,26 @@ During the above process, the user agent SHOULD show some UI to the user to guid
 authorizing an authenticator with which to complete the operation.
 </div>
 
+### Store an existing credential - PublicKeyCredential's `[[Store]](credential)` method ### {#storeCredential}
+
+<div link-for-hint="PublicKeyCredential/[[Store]](credential)">
+
+The <dfn for="PublicKeyCredential" method>\[[Store]](credential)</dfn> method is not supported
+for Web Authentication's {{PublicKeyCredential}} type, so it always returns an error.
+
+Note: This algorithm is synchronous; the {{Promise}} resolution/rejection is handled by
+{{CredentialsContainer/store()|navigator.credentials.store()}}.
+
+This method accepts a single argument:
+
+<dl dfn-type="argument" dfn-for="PublicKeyCredential/[[Store]](credential)">
+    :   <dfn>credential</dfn>
+    ::  This argument is a {{PublicKeyCredential}} object.
+</dl>
+
+When this method is invoked, the user agent MUST execute the following algorithm:
+
+1. Return a {{DOMException}} whose name is "{{NotSupportedError}}", and terminate this algorithm
 
 ### Platform Authenticator Availability - PublicKeyCredential's `isPlatformAuthenticatorAvailable()` method ### {#isPlatformAuthenticatorAvailable}