Add node version requirements to documentation

[Dadams2]

I was attempting to install lsp-pyright on a centos7 machine without root privileges. For me the default versions were:

• node v6.16.0
• npm v3.10.10

When attempting to build pyright in doom emacs I kept getting
Syntax Error: Unexpected Token *

Updating the node version to 14 fixed this as I looked #35 which mentioned downgrading to node 14 was required on mac.

I am quite new to contributing so my question is would you be open to adding a quick section in the readme about node requirements as they are not obvious on microsofts documentation either?

I would be happy to investigate what versions work and don't.