doc: add benchmark who-to-CC info

Add benchmark to "Who to CC". Also, alphabetized the only
non-alphabetized subsystem.
PR-URL: #7604
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

Author: Trott

doc/onboarding-extras.md

@@ -2,6 +2,8 @@
 
 | subsystem | maintainers |
 | --- | --- |
+|`benchmark/*`|@nodejs/benchmarking, @mscdex|
+|`bootstrap_node.js`|@fishrock123|
 |`lib/assert`|@nodejs/testing|
 |`lib/buffer`|@nodejs/buffer|
 |`lib/child_process`|@bnoordhuis, @cjihrig|
@@ -16,7 +18,6 @@
 |`lib/timers`|@fishrock123, @misterdjules|
 |`lib/util`|@bnoordhuis, @cjihrig, @evanlucas|
 |`lib/zlib`|@addaleax, @bnoordhuis, @indutny|
-|`bootstrap_node.js`|@fishrock123|
 |`src/async-wrap.*`|@trevnorris|
 |`src/node_crypto.*`|@nodejs/crypto|
 |`test/*`|@nodejs/testing|