Add link to issuer discovery spec on error

Author: dmitrizagidulin

lib/models/authenticator.js

@@ -327,7 +327,8 @@ class TlsAuthenticator extends Authenticator {
           return this.accountManager.userAccountFrom({ webId, username: webId, externalWebId: true })
         }
 
-        throw new Error(`This server is not the authorized provider for Web ID ${webId}`)
+        throw new Error(`This server is not the authorized provider for Web ID ${webId}. 
+        See https://github.com/solid/webid-oidc-spec#authorized-oidc-issuer-discovery`)
       })
   }
 }