Readme: use table for metrics

README.md

 # Nexenta Exporter for Prometheus
 This Prometheus exporter uses the Nexenta API to provide metrics for JBODs, volumes and license informations.
 
-Currently implemented metrics:
-* `nexenta_up`
-* `nexenta_scrape_failures_total`
-* `nexenta_jbod_status`
-* `nexenta_jbod_temp`
-* `nexenta_jbod_voltage` 
-* `nexenta_jbod_slot_status`
-* `nexenta_volume_online`
-* `nexenta_license_days_left`
+#### Currently implemented metrics:
+Metric | Type | Description
+--- | --- | ---
+`nexenta_up` | gauge | Is the Nexenta API reachable (0/1)
+`nexenta_scrape_failures_total` | counter |Number of errors while scraping the Nexenta API
+`nexenta_jbod_status` | gauge | Status of JBOD (0/1)
+`nexenta_jbod_temp` | gauge | Temperature of JBOD (°C)
+`nexenta_jbod_voltage` | gauge | Voltage of JBOD (Volts)
+`nexenta_jbod_slot_status` | gauge | Status of JBOD slot (0/1)
+`nexenta_volume_online` | gauge | Is the volume online (0/1)
+`nexenta_license_days_left` | gauge | License days left