Explicitly warn people about nodejs usage in README

[antonkulaga]

Despite the name, python-sdk uses nodejs server underneath and it is never mentioned anywhere! People have to struggle not understanding why in a python-sdk they get obscure nodejs errors. I believe users deserve to know about node requirements as it is super-contr-intuitive to expect node in python-sdk

[Kludex]

What error?

The Python SDK doesn't have any node.js underneath.