Update README.md to encourage yarn instead of npm?

[benjaminapetersen]

I believe the community is leaning towards yarn now, but curious if the patternfly react crew has a pref.

[priley86]

we are using yarn in products...but npm in pf-react (based on previous direction). I think now is a good time to raise this again though. Do you mind opening an issue in PF-React? ... i'd also be fine moving this repo to yarn...

[benjaminapetersen]

Yup, no prob!