Add in note to ensure that the _ is not left out. Repeated rookie mistake

Writing-a-Plugin:-Directives.md

@@ -89,6 +89,8 @@ Finally, don't forget to import your plugin's package! Caddy must import your pl
 import _ "your/plugin/package/here"
 ```
 
+*Please Note:* The `_` after import is required.
+
 That's it! Build caddy with your plugin, then write a Caddyfile with your new directive to see it in action.
 
 *If you are writing an HTTP middleware, [continue to the next page](https://github.com/mholt/caddy/wiki/Writing-a-Plugin:-HTTP-Middleware).*