add cheat for sidenotes, as I don't want to look it up every time..

BLOG.md

@@ -38,3 +38,14 @@ image: URL of an image to show along with the post
 imagealt: Alt text for the image
 
 [twitter card reference](https://developer.twitter.com/en/docs/twitter-for-websites/cards/overview/markup)
+
+Side notes
+----------
+
+Use the following, replace `sn-demo` w/ unique id.
+
+```
+<label for="sn-demo" class="margin-toggle sidenote-number"></label>
+<input type="checkbox" id="sn-demo" class="margin-toggle"/>
+<span class="sidenote"></span>
+```