react-native windows doesn't work after installing windows plug-in using npm

[kmelmon]

Our Getting Started guide says that we still support installing rnpm-plugin-windows via npm but it is currently broken.

REPRO STEPS:
react-native init myproject --version 0.60.0
cd myproject
npm install --save rnpm-plugin-windows
react-native windows --template vnext

RESULT
warn Package rnpm-plugin-windows has been ignored because it contains invalid configuration. Reason: Cannot find module '@react-native-community/cli/build/tools/generator/promptSync'
error Unrecognized command "windows".

[kmelmon]

This may be a lack of proper dependency declarations in package.json. Yarn apparently works due to hoisting up to root. We should try to fix this, if not then update docs to use yarn.

[chrisglein]

If we can't figure this out quickly let's update the steps to require yarn.
@kikisaints as an FYI for that