Added Mozilla-compliant AST specification

Generic ECMAScript+JSX parsers and some other tools use extended Mozilla AST format as intermediate representation.

This PR describes those extensions for new spec consumers that want to work with AST.
Let make the AST a stand-alone document in the same repo that we link to, from the other one. We can use strong words to encourage everyone to follow. If/when the Mozilla's AST is in ECMAScript we can merge the documents. I think for now it's clearer that README.md extends ECMA-262 and AST.md extends Mozilla's AST.