remove reference to green cell highlighter

because that has become blue in JupyterLab

Also added image at bottom

Author: fangohr

index.ipynb

@@ -54,7 +54,7 @@
     "\n",
     "### Editing code\n",
     "\n",
-    "If you want to *change* the code in the currently highlighted cell, you need to press `ENTER`. (You have now entered the [Editing mode](https://jupyter-notebook.readthedocs.io/en/stable/examples/Notebook/Notebook%20Basics.html#Edit-mode).) Now the blue line to the left of the active cell becomes green, and the content of the cell can be edited. If you have completed your changes, and you want to execute them, use the `Shift+ENTER` short cut.\n",
+    "If you want to *change* the code in the currently highlighted cell, you need to press `ENTER`. You have now entered the [Editing mode](https://jupyter-notebook.readthedocs.io/en/stable/examples/Notebook/Notebook%20Basics.html#Edit-mode), and the content of the cell can be edited. If you have completed your changes, and you want to execute them, use the `Shift+ENTER` short cut.\n",
     "\n",
     "Note that you can also edit blocks of text (or go into edit mode for a text cell unintentionally). Just press `Shift+ENTER` to render the text again, and go back into command mode."
    ]
@@ -78,6 +78,13 @@
     "\n",
     "Enjoy!"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "![Logo, created with MayaVi](static/images/mayavi/mayavi-samp.png \"3d magnetisation vector field\")\n"
+   ]
   }
  ],
  "metadata": {