doc: update WORKING_GROUPS.md - add missing groups

Add Benchmarking and Post-Mortem workgroups as they
were missing.

Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ben Noordhuis <ben@strongloop.com>
PR-URL: #3450

WORKING_GROUPS.md

@@ -27,6 +27,8 @@ back in to the TSC.
 * [Roadmap](#roadmap)
 * [Docker](#docker)
 * [Addon API](#addon-api)
+* [Benchmarking](#benchmarking)
+* [Post-mortem](#post-mortem)
 * [Starting a Working Group](#starting-a-wg)
 * [Bootstrap Governance](#bootstrap-governance)
 * [Intl](#Intl)
@@ -219,6 +221,38 @@ Their responsibilities are:
 The current members can be found in their
 [README](https://github.com/nodejs/nan#collaborators).
 
+### [Benchmarking](https://github.com/nodejs/benchmarking)
+
+The purpose of the Benchmark working group is to gain consensus
+for an agreed set of benchmarks that can be used to:
+
++ track and evangelize performance gains made between Node releases
++ avoid performance regressions between releases
+
+Its responsibilities are:
+
++ Identify 1 or more benchmarks that reflect customer usage.
+   Likely need more than one to cover typical Node use cases
+   including low-latency and high concurrency
++ Work to get community consensus on the list chosen
++ Add regular execution of chosen benchmarks to Node builds
++ Track/publicize performance between builds/releases
+
+### [Post-mortem](https://github.com/nodejs/post-mortem)
+
+The Post-mortem Diagnostics working group is dedicated to the support
+and improvement of postmortem debugging for Node.js. It seeks to
+elevate the role of postmortem debugging for Node, to assist in the
+development of techniques and tools, and to make techniques and tools
+known and available to Node.js users.
+
+Its responsibilities are:
+
++ Defining and adding interfaces/APIs in order to allow dumps
+  to be generated when needed
++ Defining and adding common structures to the dumps generated
+  in order to support tools that want to introspect those dumps
+
 ## Starting a WG
 
 A Working Group is established by first defining a charter  that can be