Add advice regarding moving from v1 to v2 to README.

[vpratz]

As we do not have full feature parity yet, I think it is important to be up-front with the functionality that is still missing, to avoid confusion and disappointment. As we now have the 2.0.0 version on PyPI, some advice on whether to upgrade or not might be helpful.

This PR adds the list of missing functionality from #388, as well as some additional considerations. Feel free to edit, but I think getting something like this out soon would be good.

Closes #388.

[vpratz]

I have added a short phrase regarding the v1 docs, and a link to the migration advice so people can find it.

[stefanradev93]

@paul-buerkner What do you think? I believe we can shorten some of the description, as most of the old functionality is probably not relevant. The two level model is probably the most important gap which I intend to close soon.

[paul-buerkner]

I like it. About shortening, do you mean the new edits of this PR or older stuff in the vignette? Having some more details cannot hurt I would say. Why would you want to shorten it?

[paul-buerkner]

Edit: I now realize this is supposed to be part of the readme not the vignette. Yes, I agree we should shorten it a lot.
Or perhaps better, put the caution sign to the top but all the details down somewhere in the FAQ (and a link to it at the top). It is not helpful for a readme file if I have to read a lot about old stuff without seeing all the cool new stuff first.

[vpratz]

I have moved the details to the FAQ. Any thoughts on how to best highlight the section in the top part? I'm not sure whether it needs a separate heading, or some highlighting is enough.
I can see that presenting new features is the exciting part with the new release, but I think at least in the next few weeks we should also focus on our existing user base, so that they do not get the impression that there is a new version and "nothing works anymore".
As the new version is on PyPI now, and some users may upgrade by accident, I think having this information somewhere prominent soon is important.

[vpratz]

Again, feel free to edit as you see fit. If you are happy with the text, I will copy the FAQ section to the FAQ on the website as well...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

[paul-buerkner]

Looks good to me. Thank you! From my side, we can merge as is.