Add README

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`