Add comment system documentation

plugins/pelican_comments/README.md

@@ -50,6 +50,21 @@ authors and date/time. For example:
 
 It doesn't matter what the files are named.
 
+# How to mention a user ?
+
+Due to the absence of javascript in the comment system.
+Mentioning a user can be difficult, however we just have
+to put a `<a>` tag at the beginning of the comment,
+Here I show you an example:
+
+    <a href="#comment-saravia-20191118-20:14:50">@saravia</a>
+    Reply message.
+
+where:
+
+- `#comment-saravia-20191118-20:14:50` is the comment `id` of the
+  user to respond.
+
 # Example folder structure
 
 	.