document new heuristics

test/README.md

@@ -151,6 +151,22 @@ Our tests run on different browsers to increase the coverage:
 - [Headless Chrome](https://chromium.googlesource.com/chromium/src/+/lkgr/headless/README.md)
 - Chrome, Firefox, Safari, and Edge thanks to [BrowserStack](https://www.browserstack.com)
 
+##### BrowserStack
+
+We only use BrowserStack for non-PR commits to save ressources.
+Browserstack rarely reports actual issues so we only use it as a stop-gap for releases not merges.
+
+To force a run of BrowserStack on a PR you have to run the pipeline with `browserstack-force` set to `true`.
+For example, you've opened a PR with the number 64209 and now after everything is green you want to make sure the change passes all browsers:
+
+```bash
+curl --request POST \
+  --url https://circleci.com/api/v2/project/gh/mui-org/material-ui/pipeline \
+  --header 'content-type: application/json' \
+  --header 'Circle-Token: $CIRCLE_TOKEN' \
+  --data-raw '{"branch":"pull/64209/head","parameters":{"browserstack-force":true}}'
+```
+
 ### Browser API level
 
 In the end, components are going to be used in a real browser.