FAQ in README

[LarsKue]

Commonly asked questions should be added in their own section in the README.

[paul-buerkner]

Let's use this issue to gather some initial FAQs for the Bayesflow 2.0 version. I will start:

• What are the main differences between Bayesflow 2.0+ and earlier versions of the library?
• Which deep learning backend shall I best use with Bayesflow?

[LarsKue]

Users also requested a troubleshooting section in the form of

Q: My network does X, how do I fix it?
A: Use gradient clipping / deeper networks / bigger batch size / ...

[paul-buerkner]

I have added some basic FAQ in the readme. We can just add more stuff as it comes up.

With respect to network trouble shooting, the answers may be a bit longer and require code, so I suggest we add them to some of our notebooks. Either new ones, focussed on fixing issues, or old ones if the issues are there too.