Update README.md

Author: mdjastrzebski

README.md

@@ -29,6 +29,7 @@ MIT
 
 - [Swift Regex Builder API docs](https://developer.apple.com/documentation/regexbuilder)
 - [Swift Evolution 351: Regex Builder DSL](https://github.com/apple/swift-evolution/blob/main/proposals/0351-regex-builder.md)
+- [ECMAScript Regular Expression BNF Grammar](https://262.ecma-international.org/7.0/#sec-regular-expressions)
 
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