Add M3DB to list of remote storages (#1138)

Signed-off-by: Rob Skillington <r@uber.com>

content/docs/operating/integrations.md

@@ -41,6 +41,7 @@ data volumes.
   * [Graphite](https://github.com/prometheus/prometheus/tree/master/documentation/examples/remote_storage/remote_storage_adapter): write
   * [InfluxDB](https://docs.influxdata.com/influxdb/v1.4/supported_protocols/prometheus): read and write
   * [IRONdb](https://github.com/circonus-labs/irondb-prometheus-adapter): read and write
+  * [M3DB](https://m3db.github.io/m3/integrations/prometheus): read and write
   * [OpenTSDB](https://github.com/prometheus/prometheus/tree/master/documentation/examples/remote_storage/remote_storage_adapter): write
   * [PostgreSQL/TimescaleDB](https://github.com/timescale/prometheus-postgresql-adapter): read and write
   * [SignalFx](https://github.com/signalfx/metricproxy#prometheus): write