add hint about broken URLs when jupyterlab is missing in truly-minimal example

[N-Coder]

see jupyterhub/mybinder.org-user-guide#240

[minrk]

closing in favor of #5

[N-Coder]

@minrk note that these two PRs want to merge into two different branches:
#4: 1 commit into binder-examples:truly-minimal from N-Coder:patch-1
#5: 2 commits into binder-examples:master from N-Coder:patch-2
If we don't merge this PR, the truly-minimal branch, which is also referenced from the docs as alternative, truly-minimal example, will remain broken.

---

See here:

The absolute bare minimum for this is to set HOME to /tmp so that it's writable,
which would make the shortest, smallest Dockerfile that works on Binder:

FROM python:3.7-slim
RUN pip install --no-cache notebook
ENV HOME=/tmp

which you can try out:

And the binder badge points to https://mybinder.org/v2/gh/binder-examples/minimal-dockerfile/truly-minimal, i.e. said branch of this repo.

[minrk]

Thanks for that clarification! I forgot about the other branch, I'll make sure both install both notebook and lab, which should be considered the truly minimal baseline to be "supported", even if it may technically work for now.