add instructions for getting the code if there is no token

src/gog.rs

@@ -212,6 +212,8 @@ impl<'a> Gog<'a> {
     fn get_code(&self) -> Result<String, GogError> {
         let auth_uri = self.auth_uri(self.client_id.as_str(), self.redirect_uri.as_str());
 
+        println!("Open the following url in a browser, login to your account and paste the \
+                  resulting code parameter.");
         println!("{}", auth_uri);
 
         let mut code = String::new();