[readme] document v2.0.0 requirements

[pkra]

The readme could mention that mathjax-node now requires node 6+ and jsdom 10+.

The breaking changes from v1 (and v2) can probably move to the bottom. Related libraries probably as well.

[dpvc]

Yes, I saw that when I checked the 2.0 release on npmjs. Was planning to do something about it today. Thanks for checking.

[dpvc]

I've updated the README, and will tag it as a new release so that npmjs will grab the new README.

[pkra]

Thanks, Davide. All fixed.