Update README requirements inline with Prereqs page

[Jakeeyturner]

Description

The requirements in the README need updating as they are inconsistent with the Prerequsites page.
We should update this section to be clearer about the required and optional dependencies across different OS's.

Expected Behavior

Actual Behavior

Possible Fix

Steps to Reproduce

Existing issues

• Stack Overflow issues
• GitHub Issues

Log File

Your Environment

• IBM Blockchain Platform Extension Version:
• Visual Studio Code version:
• Environment name and version (e.g. Node.js v8.12.0, npm v6.4.1):
• Operating System and version:
• Link to your project: