add comments about custom plugins

Author: Ilya-Valasiuk

serverless.yml

@@ -10,6 +10,9 @@ provider:
 
 plugins:
   - serverless-finch
+  # 'serverless-single-page-app-plugin' is a custom plugin that located .serverless_plugins folder.
+  # Existing plugin (https://www.npmjs.com/package/serverless-single-page-app-plugin) doesn't have invalidate cache feature that often used during CI/CD events.
+  # How to build your own plugins: https://www.serverless.com/framework/docs/providers/aws/guide/plugins#service-local-plugin
   - serverless-single-page-app-plugin
 
 custom: