Give a tip about not forgetting @ in curl command.

src/docs/markdown/api-tutorial.md

@@ -73,6 +73,11 @@ Then upload it:
 	-d @caddy.json
 </code></pre>
 
+<aside class="tip">
+	Make sure you dont forget the @ in front of your filename, this tells curl you are sending a file.
+</aside>
+
+
 <aside class="complete">Give Caddy a config</aside>
 
 We can verify that Caddy applied our new config with another GET request:
@@ -105,6 +110,8 @@ Save the config file, then update Caddy's active configuration by running the sa
 	-d @caddy.json
 </code></pre>
 
+
+
 <aside class="complete">Replace active config</aside>
 
 For good measure, verify that the config was updated: