add slotracker.com

README.md

@@ -433,6 +433,8 @@ Contributions are always welcome!
 * [How We Use Sloth to do SLO Monitoring and Alerting with Prometheus](https://mattermost.com/blog/sloth-for-slo-monitoring-and-alerting-with-prometheus/)
 * [SLI Deep Dive](https://medium.com/site-reliability-engineering-leadership/sli-deep-dive-cae92bd90a79)
 * [Measuring Reliability in GCP: Step By Step SLO creation guide using Cloud Operation Sandbox](https://medium.com/google-cloud/measuring-reliability-in-gcp-step-by-step-slo-creation-guide-using-cloud-operation-sandbox-99043bd0e70f)
+* [SLO tracker](https://slotracker.com/)
+
 
 ## Performance
 * [Performance Checklists for SREs](https://www.usenix.org/conference/srecon16/program/presentation/gregg)