Update project README installation instructions

I think we can update the README file.

Here we use `will` as in the future, but I think we should change it to the present.

>This plugin will be capable of executing code fragments via IPython kernel as currently supported by Jupyter.

Under the Python dependencies, we need to specify the Python version. I can see JDK 8, but no mention of Python version.

Also probably just need a list of dependencies. So users can choose to use `pip`, `poetry`, `conda`, etc. But probably also good to specify the version if we know what version is required.

The section about contributing to the plugin is probably better in CONTRIBUTING.md, in a separate file.

Finally, the instructions start with the Python dependencies, then go on some configuration. Then come back to JDK and more dependencies in "Guideline for setup". We should have a single section for the setup/installations.