Add stage 0: Ergonomic brand checks for Private Fields

stage-0-proposals.md

@@ -24,6 +24,7 @@ Stage 0 proposals are either
 | [`deprecated`][deprecated]                                         | James M Snell                         | James M Snell                         |                                   |
 | [`as` destructuring patterns][as-patterns]                         | Kat Marchán                           | Kat Marchán                           |                                   |
 | [`Symbol.thenable`][symbol-thenable]                               | Gus Caplan                            | Jordan Harband<br />Myles Borins      | [May 2018][symbol-thenable-notes] |
+| [Ergonomic brand checks for Private Fields][private-fields-in-in]  | Jordan Harband                        | Jordan Harband                        |
 
 See also the [active proposals](README.md), [stage 1 proposals](stage-1-proposals.md), [finished proposals](finished-proposals.md), and [inactive proposals](inactive-proposals.md) documents.
 
@@ -53,3 +54,4 @@ See also the [active proposals](README.md), [stage 1 proposals](stage-1-proposal
 [decimal-notes]: https://github.com/tc39/notes/blob/master/meetings/2017-11/nov-29.md#9ivb-decimal-for-stage-0
 [symbol-thenable]: https://github.com/devsnek/proposal-symbol-thenable
 [symbol-thenable-notes]: https://github.com/tc39/notes/blob/def2ee0c04bc91612576237314a4f3b1fe2edaef/meetings/2018-05/may-24.md#symbolthenable-for-stage-1-or-2
+[private-fields-in-in]: https://github.com/ljharb/proposal-private-fields-in-in