Clarification about the relation to cling

[naweiss]

The documentation of cling states:

Cling comes with a Jupyter kernel. After building cling, install Jupyter and cling's kernel by following the README.md in tools/Jupyter.

This projects is also a Jupyter kernel for C++ based on the cling. So, it would be helpful to claritfy in the README file, what is exactly the difference between the "builtin" cling kernel and the kernel used in this project.
In other words, what is the motivation of using/creating this project.

[JohanMabille]

The kernel provided by ROOT is based on the wrapper kernel approach, meaning it depends on ipykernel and thus on the Python runtime.

This kernel is based on xeus, a native implementation of the Jupyter Kernel protocol. No dependency on ipykernel nor any Python part of the Jupyter ecosystem, and it has proven to be more stable on the long run.

[naweiss]

Thanks, wouldn't it be worth to mention it somehow in the doc?

[JohanMabille]

Yes, would you like to open a PR?

[naweiss]

I added a new section which might be an overkill, but I think it makes it clearer for new users why should they prefer xeus-cling over cling - #478.