docs: Update README

README.md

@@ -143,6 +143,18 @@ Playwright is run on every successful deployment of a branch. Screenshots are ma
 A special [test page](http://localhost:3000/en/__test) shows all the charts that are screenshotted.
 Those charts configurations are kept in the repository.
 
+## GraphQL performance tests
+
+The project uses a combination of [k6](https://k6.io) and [Grafana](https://grafana.com) with [Prometheus](https://k6.io/docs/results-output/real-time/prometheus-remote-write/) for GraphQL performance testing.
+
+### Automation
+
+To ensure constant monitoring of the performance of selected GraphQL queries, the performance tests are run [once an hour](https://github.com/visualize-admin/visualization-tool/blob/main/.github/workflows/performance-tests.yml) against each environment of the application. The results are then sent to the governmental Grafana dashboards.
+
+### How to add or modify the tests
+
+To add or modify the performance tests, go to the [k6/performance-tests](https://github.com/visualize-admin/visualization-tool/tree/main/k6/performance-tests) folder. The GitHub Action is generated by running the `yarn run github:codegen` command; be sure to run it after modifying the [generate-github-action.js](`https://github.com/visualize-admin/visualization-tool/blob/main/k6/performance-tests/generate-github-action.js`) file.
+
 ## Load tests
 
 The project uses [k6](https://k6.io) for load testing.