docs: add readme

Signed-off-by: Sven Gerber <github@gerber.uno>

README.md

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+# Alertmanager middleware for Uptime Kuma push monitor
+![Docker Image Version (latest semver)](https://img.shields.io/github/v/release/natrontech/alertmanager-uptime-kuma-push)
+![Go Version](https://img.shields.io/github/go-mod/go-version/natrontech/alertmanager-uptime-kuma-push)
+![GitHub Workflow Status](https://img.shields.io/github/actions/workflow/status/natrontech/alertmanager-uptime-kuma-push/release.yml)
+
+## What is this?
+This binary allows you to use Uptime Kuma as a [Dead Mans Snitch](https://deadmanssnitch.com/) for your Prometheus Alertmanager.
+If your Alertmanager stops sending alerts, Uptime Kuma will send you a notification.
+
+## Why does this need an extra middleware?
+Alertmanager allows you to configure webhooks to send alerts to.
+All webhook alerts from Alertmanager are sent using the HTTP POST method.
+Uptime Kuma has a Push Monitor, that listens only for HTTP GET requests.
+This middleware listens for POST requests on port 8081 and the `/push` path and forwards them to Uptime Kuma as plain GET requests.
+
+## How to use
+Configure this middleware as a sidecar container in your Alertmanager deployment.
+Then send alerts to this middleware to forward them to Uptime Kuma.
+
+The binary requires the following environment variables:
+| Variable | Description |
+| --- | --- |
+| `UPTIME_KUMA_URL`| The URL to your Uptime Kuma instance. This URL should be the push monitor URL. |
+
+### Example with kube-prometheus-stack
+Configure a push monitor in your Uptime Kuma instance.
+
+![Uptime Kuma push monitor](docs/uptime_kuma_push.png)
+
+Add a config map with the Uptime Kuma push URL.
+```yaml
+apiVersion: v1
+kind: Secret
+metadata:
+    name: uptime-kuma-url
+    namespace: monitoring
+type: Opaque
+stringData:
+    UPTIME_KUMA_URL: "https://uptime-kuma.example.com/api/push/XXXXXXX"
+```
+Add this middleware as a sidecar container to your Alertmanager deployment.
+```yaml
+alertmanager:
+    alertmanagerSpec:
+    # Sidecar middleware to push watchdog alerts to uptime-kuma
+        containers:
+        - name: alertmanager-uptime-kuma-push
+            image: ghcr.io/natrontech/alertmanager-uptime-kuma-push:0.0.2
+            ports:
+            - containerPort: 8081
+            envFrom:
+            - secretRef:
+                name: uptime-kuma-url
+```
+Add a new receiver for the Watchdog alert to your Alertmanager configuration.
+We don't send resolved because the Watchdog alert is always firing.
+```yaml
+route:
+    routes:
+        - receiver: Dead Mans Snitch Uptime Kuma
+          matchers:
+            - alertname =~ "InfoInhibitor|Watchdog"
+          group_wait: 0s
+          group_interval: 1m
+          repeat_interval: 50s
+receivers:
+    - name: Dead Mans Snitch Uptime Kuma
+      webhook_configs:
+        - url: http://localhost:8081/push
+          send_resolved: false
+```