adds a readme.

README.md

+# Jupyterlab PostgreSQL Image
+
+A jupyterlab image with postgresql installed and started.
+A database `demodb` is created for user `jovyan`.
+
+```bash
+WORKDIR=$HOME/JUPYTER_WORK
+docker run --rm -it \
+        --user root \
+        --name ${PWD##*/} \
+        --volume $WORKDIR:/home/jovyan/work \
+    --publish 8888:8888 \
+    --env NB_UID=$UID \
+    brunoe/jupyter-db-pg:develop start-notebook.sh
+```