Improve error message

README.md

@@ -75,7 +75,7 @@ Generators
 ----------
 
 ### API Keys
-
+<!-- This anchor name is linked to from  `templates/shopify_app.rb.tt` so beware of changing -->
 Before running the generators, you'll need to ensure your app can read the Shopify environment variables `SHOPIFY_API_KEY` and `SHOPIFY_API_SECRET`.
 
 A common approach is to use the [dotenv-rails](https://github.com/bkeepers/dotenv) gem, along with an `.env` file in the following format:

lib/generators/shopify_app/install/templates/shopify_app.rb.tt

@@ -1,8 +1,8 @@
 unless defined? Rails::Generators
   ShopifyApp.configure do |config|
     config.application_name = "<%= @application_name %>"
-    config.api_key = ENV.fetch('SHOPIFY_API_KEY', '').presence || raise('Missing SHOPIFY_API_KEY')
-    config.secret = ENV.fetch('SHOPIFY_API_SECRET', '').presence || raise('Missing SHOPIFY_API_SECRET')
+    config.api_key = ENV.fetch('SHOPIFY_API_KEY', '').presence || raise('Missing SHOPIFY_API_KEY. See https://github.com/Shopify/shopify_app#api-keys')
+    config.secret = ENV.fetch('SHOPIFY_API_SECRET', '').presence || raise('Missing SHOPIFY_API_SECRET. See https://github.com/Shopify/shopify_app#api-keys')
     config.old_secret = "<%= @old_secret %>"
     config.scope = "<%= @scope %>" # Consult this page for more scope options:
                                    # https://help.shopify.com/en/api/getting-started/authentication/oauth/scopes