Describe how to test the Alertmanager interface

Author: eikendev

docs/docs/developer-guide.md

@@ -32,3 +32,44 @@ Running tests for all PushBits module is done like this:
 ```bash
 make test
 ```
+
+## Testing the Alertmanager interface
+
+Setting up a local [Alertmanager](https://prometheus.io/docs/alerting/latest/alertmanager/) instance is done quickly using Docker or Podman.
+We first create the file `config.yml` that tells Alertmanger where to send alerts.
+Don't forget to replace `<PB_TOKEN>` with your application's token.
+```yaml
+route:
+  receiver: default
+
+receivers:
+  - name: default
+    webhook_configs:
+      - url: http://localhost:8080/alert?token=<PB_TOKEN> # Replace <PB_TOKEN> with your application's token.
+```
+
+We mount this file into a container running Alertmanager.
+For convenience, let's use this Makefile:
+```make
+CONTAINER_NAME := alertmanager
+IMAGE_NAME := quay.io/prometheus/alertmanager
+
+.PHONY: up
+up:
+	podman run --name $(CONTAINER_NAME) --rm --network host -v $(PWD)/config.yml:/etc/alertmanager/alertmanager.yml:Z $(IMAGE_NAME)
+
+.PHONY: down
+down:
+	podman container stop $(CONTAINER_NAME)
+
+.PHONY: send
+send:
+	amtool alert add alertname=testmessage --annotation=title='My Title' --annotation=message='My Message'
+
+.PHONY: setup
+setup:
+	go install github.com/prometheus/alertmanager/cmd/amtool@latest
+```
+
+So first you would install the client using `make setup`, followed by starting Alertmanager with `make up`.
+When PushBits is running, a `make send` would create a new alert in Alertmanager, which is then passed to PushBits via the defined webhook.