Add RegExp Lookbehind Assertions Proposal

Advanced to Stage 2 at November 2016 meeting.

README.md

@@ -20,6 +20,7 @@ Proposals follow [this process document](https://tc39.github.io/process-document
 |   | [Promise.prototype.finally](https://github.com/tc39/proposal-promise-finally)                             | Jordan Harband                     | 2 |
 |   | [Class and Property Decorators](http://tc39.github.io/proposal-decorators/)                               | Yehuda Katz and Brian Terlson      | 2 |
 |   | [Legacy RegExp features in JavaScript](https://github.com/tc39/proposal-regexp-legacy-features)           | Mark Miller, Claude Pache          | 2 |
+|   | [RegExp Lookbehind Assertions](https://github.com/tc39/proposal-regexp-lookbehind)                        | Daniel Ehrenberg                   | 2 |
 |   | [RegExp Unicode Property Escapes](https://github.com/tc39/proposal-regexp-unicode-property-escapes) | Brian Terlson, Daniel Ehrenberg, Mathias Bynens | 2 |
 |   | [Private Fields](https://github.com/tc39/proposal-private-fields)                                         | Daniel Ehrenberg                   | 2 |
 |   | [Date.parse fallback semantics](https://github.com/mrrrgn/proposal-date-time-string-format)               | Morgan Phillips                    | 1 |