Add development README for JupyterLab extension (#4269)

ts/jupyter_extension/README.md

+NNI is under development to support JupyterLab.
+You can install this extension to preview the feature.
+
+Currently you can view NNI web UI inside JupyterLab.
+
+## Install ##
+
+To preview the extension, you need to have `nni` and `jupyterlab` installed at first:
+
+```
+$ pip install nni jupyterlab
+```
+
+Then run following command to register extension:
+
+```
+$ nnictl jupyter-extension install
+```
+
+It does not have prompt message. Exit without error means success.
+
+## Run ##
+
+For now, the extension does not support creating experiment, so you have to create one with nnictl:
+
+```
+$ nnictl create --config <experiment-directory>/config.yml
+```
+
+And you need to launch JupyterLab:
+
+```
+$ jupyter lab --ip=0.0.0.0
+```
+
+Following JupyterLab's guide to open its web page, you should find an NNI icon.
+Click the icon and it will open NNI web UI for your running experiment.
+
+## Uninstall ##
+
+To uninstall (or more accurately, unregister) the extension, run following command:
+
+```
+$ nnictl jupyter-extension uninstall
+```
+
+## Known Issues ##
+
+The JupyterLab extension is under development and there are many issues need to fix before public announcement:
+
+  * Clicking a link in experiment management page will open it outside JupyterLab. To fix it will need modify in web UI.
+  * Downloading log file might not work.
+  * Post requests (update experiment config) might not work.