Update CONTRIBUTING.md

Author: Minishlink

.github/CONTRIBUTING.md

@@ -2,6 +2,18 @@
 WebPush is an open source library.
 Feel free to contribute by submitting a pull request or creating (and solving) issues!
 
+## Requirements before submitting a pull request
+
+The CI used to check that push notifications can still be sent after the proposed code changes thanks to [web-push testing service](https://www.npmjs.com/package/web-push-testing-service). Unfortunately, this package doesn't work anymore and I don't have the available time to fix it. We can't accept new PR without being sure that the code changes doesn't break anything. So, for a PR to be accepted, it is now requested to have one of these 3 solutions :
+
+1. You fix web-push-testing-service completely, but it's very time consuming
+2. You fix web-push-testing-service but only for stable version of Chrome, looks a bit more promising but still you'll need some time
+3. You don't fix web-push-testing-service, but you add a video that shows that the PR changes work as expected, and that core feature (sending a simple push with payload) works. For example a video showing a local test passing with a push notification on Chrome. Please make it enough clear for me to be 100% sure that it's ok to merge by looking at your code and the video.
+
+Please don't make huge pull requests that introduce too many changes too.
+
+Thanks!
+
 ## Installing a mock push service
 
 Before running tests, you'll need to install the [web-push testing service](https://www.npmjs.com/package/web-push-testing-service):