docs: link to fetch test coverage issue (nodejs#1191)

Refs: nodejs#951

README.md

@@ -164,7 +164,9 @@ Implements [fetch](https://fetch.spec.whatwg.org/#fetch-method).
 
 Only supported on Node 16.5+.
 
-This is [experimental](https://nodejs.org/api/documentation.html#documentation_stability_index) and is not yet fully compliant with the Fetch Standard. We plan to ship breaking changes to this feature until it is out of experimental.
+This is [experimental](https://nodejs.org/api/documentation.html#documentation_stability_index) and is not yet fully compliant with the Fetch Standard.
+We plan to ship breaking changes to this feature until it is out of experimental.
+Help us improve the test coverage by following instructions at [nodejs/undici/#951](https://github.com/nodejs/undici/issues/951).
 
 Basic usage example: