Clarify feature support

[Thom1729]

I'm curious as to what JavaScript features this package supports, above and beyond the core syntax.

The front page bills this syntax as "More accurate to the JavaScript specification". If you're referring to standard functionality, then the core syntax should probably be updated to incorporate it. If you're referring to experimental features, then it might be helpful to state which experimental features this syntax supports, or in general what your policy is in regards to experimental features (e.g. all Stage 2 and higher).

[Thom1729]

@JoshuaWise ping