Document promise polyfill needed for IE

[jasongrout]

If the user wants things to work on IE 11, they need a promise polyfill, such as es6-promise. This should be documented somewhere.

(Inspired from sagemath/sagecell#466).

[willingc]

@jasongrout Do you think this would be better here or in jupyterlab documentation?

If here, at end of README or a new technical-note.md?

[blink1073]

cf #167