Add link to decorators proposal (#606) [skip ci]

README.md

@@ -134,7 +134,7 @@ require("babylon").parse("code", {
 | `typescript` ([repo](https://github.com/Microsoft/TypeScript)) | `var a: string = "";` |
 | `doExpressions` | `var a = do { if (true) { 'hi'; } };` |
 | `objectRestSpread` ([proposal](https://github.com/tc39/proposal-object-rest-spread)) | `var a = { b, ...c };` |
-| `decorators` (Stage 1) and `decorators2` (Stage 2) | `@a class A {}` |
+| `decorators` (Stage 1) and `decorators2` (Stage 2 [proposal](https://github.com/tc39/proposal-decorators)) | `@a class A {}` |
 | `classProperties` ([proposal](https://github.com/tc39/proposal-class-public-fields)) | `class A { b = 1; }` |
 | `classPrivateProperties` ([proposal](https://github.com/tc39/proposal-private-fields)) | `class A { #b = 1; }` |
 | `exportExtensions` ([proposal 1](https://github.com/leebyron/ecmascript-export-default-from)), ([proposal 2](https://github.com/leebyron/ecmascript-export-ns-from)) | Proposal 1: `export v from "mod"` Proposal 2: `export * as ns from "mod"` |