updated README with origin of this repo

README.md

+*IMPORTANT NOTE*
+================
+
+this repo is a private fork based on top of https://github.com/camptocamp/prometheus-puppetdb-exporter/pull/12
+
+added:
+
+- `puppetdb_status` metric like in https://github.com/voxpupuli/puppet-prometheus_reporter/blob/cee6ebd542f7a7fb2d7abdfa923cc9c2a356d03c/lib/puppet/reports/prometheus.rb
+
 Prometheus PuppetDB exporter
 ============================