Document required ECMAscript version

[fhammerschmidt]

I don't think we have documented the requirements on the JS version / spec at the moment.

Starting with v12, ES6 (ECMAScript 2015) is required because we are now making use of the following ES6 features:

• let keyword
• arrow functions
• spread operator

Originally posted by @cknitt in rescript-lang/rescript#7030 (comment)

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Document for both:

• v11 is mostly es5 (some stdlib functions may already have been es6)
• v12 is just es6