Mark +new as unavailable. (#18)

GoogleSignIn/Sources/Public/GoogleSignIn/GIDConfiguration.h

@@ -37,6 +37,10 @@ NS_ASSUME_NONNULL_BEGIN
 /// Identifier in the OpenID Connect ID token.
 @property(nonatomic, readonly, nullable) NSString *openIDRealm;
 
+/// Unavailable.  Please use `initWithClientID:` or one of the other initializers below.
+/// :nodoc:
++ (instancetype)new NS_UNAVAILABLE;
+
 /// Unavailable.  Please use `initWithClientID:` or one of the other initializers below.
 /// :nodoc:
 - (instancetype)init NS_UNAVAILABLE;