add X-Request-ID to docs

views/index.jade

@@ -281,6 +281,13 @@ block content
                 li
                   | <b>error</b>: This can happen, for example, when Mojang's servers are down.<br>
                   | If possible, an outdated image is served instead.
+            section
+              a(id="meta-x-request-id" class="anchor")
+              a(href="#meta-x-request-id")
+                h4 X-Request-ID
+              p
+                | The internal ID assigned to this request.<br>
+                | If you think something is wrong with your request, please <a href="#contact">contact us</a> and provide this ID.
 
           section
             a(id="meta-about-usernames" class="anchor")