add YAML info

Author: RaisinTen

doc/api/child_process.md

@@ -146,6 +146,9 @@ exec('"my script.cmd" a b', (err, stdout, stderr) => {
 <!-- YAML
 added: v0.1.90
 changes:
+  - version: v15.4.0
+    pr-url: https://github.com/nodejs/node/pull/36308
+    description: AbortSignal support was added.
   - version: v8.8.0
     pr-url: https://github.com/nodejs/node/pull/15380
     description: The `windowsHide` option is supported now.