docs: add blogpost entry by PagerTree LLC (#41)

* Add "Learn Prometheus (Visually)"
* docs: update blogpost entry in README.md

---------

Co-authored-by: Roald Nefs <info@roaldnefs.com>

README.md

@@ -105,6 +105,7 @@ Prometheus is an open-source systems monitoring and alerting toolkit.
 - [Alerting issues with Alertmanager](https://ali.sattari.me/posts/2020/alerting-issues-with-alertmanager/) - Addressing alert flapping and duplicates in Alertmanager by [Ali Sattari](https://github.com/ali-sattari).
 - [Contributing to Prometheus](https://atibhiagrawal.medium.com/contributing-to-prometheus-2bf35bd28256) - Contributing to Prometheus by Atibhi Agrawal.
 - [Simple Prometheus queries for metrics inspection](https://mkaz.me/blog/2023/simple-prometheus-queries-for-metrics-inspection/) - Overview of PromQL queries that help to identify high cardinality metrics by Michal Kazmierczak.
+- [Learn Prometheus](https://pagertree.com/learn/prometheus) - A crash course of Prometheus by PagerTree LLC.
 
 ## Deployment tools
 - [Ansitheus](https://github.com/ntk148v/ansitheus) - Ansible playbook to containerize, configure and deploy Prometheus ecosystem _by ntk148v_.