Curate list of ES2015 browsers

[redallen]

It's important to know for some PatternFly consumers to know which browser versions PatternFly supports since they might use RHEL or other long release-cycle distros. After #4046 we should target ES2015, and with that knowledge we can curate a list of supported browsers to proudly display in our README.

[redallen]

Our transpiled code (but not necessarily its dependencies) supports ES6. I don't think that information doesn't necessarily belong in a README anymore (and we added it to patternfly.org in another PR), so I'm just closing this for now.