docs: update README with GitOps commands (elastic#15928)

* ci: apm-beats-update disable notifications temporarily

* docs: update README with GitOps commands

* Apply suggestions from code review

Co-Authored-By: cachedout <mike.place@elastic.co>

Co-authored-by: cachedout <mike.place@elastic.co>

README.md

@@ -85,3 +85,18 @@ your dev environment to build Beats from the source.
 ## Snapshots
 
 For testing purposes, we generate snapshot builds that you can find [here](https://beats-ci.elastic.co/job/elastic+beats+master+multijob-package-linux/lastSuccessfulBuild/gcsObjects/). Please be aware that these are built on top of master and are not meant for production.
+
+## CI
+
+It is possible to trigger some jobs by putting a comment on a GitHub PR.
+(This service is only available for users affiliated with Elastic and not for open-source contributors.)
+
+* [beats][]
+  * `jenkins run the tests please`
+  * `jenkins run tests`
+* [apm-beats-update][]
+  * `/run apm-beats-update`
+
+
+[beats]: https://beats-ci.elastic.co/job/Beats/job/beats-beats-mbp/
+[apm-beats-update]: https://beats-ci.elastic.co/job/Beats/job/apm-beats-update/