[Docs] Add "New Features" section to README

[phated]

I think it'd be great to have a "New Features" section in the readme to help people understand all of the awesome things in gulp 4.

@stevelacy want to take a crack at this? Or anyone, really.

[yocontra]

@phated Do you have a rough idea of what you want to emphasize? I can write a top section for the README that includes new features, migration guide link, etc.

[phated]

I think a lot of stuff is in changelog.md barring any of the stuff we've done in the past 6-8 months

[stephenlacy]

I can

[yocontra]

@stevelacy If you want to do an initial outline I can do some of the fluffier stuff around it.

[phated]

Anyone want to take a swing at this?