Document variables in include tag

site/docs/templates.md

@@ -192,6 +192,19 @@ Jekyll expects all include files to be placed in an `_includes` directory at the
 root of your source directory. This will embed the contents of
 `<source>/_includes/footer.html` into the calling file.
 
+<div class="note">
+  <h5>ProTip™: Use variables as file name</h5>
+  <p>
+
+    The name of the file you wish to embed can be literal (as in the example above),
+    or you can use a variable, using liquid-like variable syntax as in
+    <code>{% raw %}{% include {{my_variable}} %}{% endraw %}</code>.
+
+    Note that unlike usual liquid variable syntax, you cannot have spaces inside the curly braces.
+
+  </p>
+</div>
+
 You can also pass parameters to an include:
 
 {% highlight ruby %}