Add note about workaround for editable user installs via pip.

[jasongrout]

Apparently a bug in pip prevents projects with pyproject.toml from having editable --user installs.

See pypa/pip#7953 and #3351

[github-actions]

👈 Launch a binder notebook on branch jasongrout/ipywidgets/usereditable

[purarue]

@jasongrout See setuptools issue, may be an issue with the -e (editable) flag, and not the user prefix

Edit: ah, no, this does seem to be an issue with --user permissions. The solution mentioned here solves a similar issue reported in setuptools recently, so I had conflated the two; my bad