docs: add a short explanation to binder notebook

Author: mariobuikhuizen

binder/empty.ipynb

@@ -1,5 +1,16 @@
 {
  "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Use the `Snippets` menu option to add snippets to this notebook.\n",
+    "\n",
+    "Snippets can be added or edited in the `snippets_symlink` directory.\n",
+    "\n",
+    "See https://github.com/QuantStack/jupyterlab-snippets for more information."
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,