Add README

desktop/km3book/README.md

+# KM3Book Docker Image
+
+This is the KM3Book Docker image which will spawn a working JupyterLab instance
+on your own computer, containing `Jpp`, `aanet v1` and `km3pipe`.
+
+It will automatically mount your `$HOME/notebooks` folder as a working
+directory where you can access all the files and notebooks, even when the
+Docker container is not running.
+
+To start up, simply launch the `startup.sh` script. The very first time, it
+needs to download a few GB of data, but consecutive runs will be faster:
+
+    startup.sh
+
+To access the JupterLab instance, click on the link shown in the terminal --
+and leave that terminal open!