added readme

README.md

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+# domain_exporter
+
+Exports the expiration time of your domains as prometheus metrics.
+
+## Running
+
+```console
+./domain_exporter -b ":9222"
+```
+
+Or with docker:
+
+```console
+docker run -p 9222:9222 caarlos0/domain_exporter
+```
+
+## Configuration
+
+On the prometheus settings, add the domain_expoter prober:
+
+```yaml
+- job_name: domain
+  scrape_interval: 1m
+  metrics_path: /probe
+  relabel_configs:
+    - source_labels: [__address__]
+      target_label: __param_target
+    - source_labels: [__param_target]
+      target_label: domain
+    - target_label: __address__
+      replacement: localhost:9222 # domain_exporter address
+  static_configs:
+    - targets:
+      - carlosbecker.com
+      - carinebecker.com
+      - watchub.pw
+```
+
+It works more or less like prometheus's
+[blackbox_exporter](https://github.com/prometheus/blackbox_exporter).
+
+## Building locally
+
+Install the needed tooling and libs:
+
+```console
+make setup
+```
+
+Run with:
+
+```console
+go run main.go
+```
+
+Run tests with:
+
+```console
+make test
+```