[docs] How to write to multiple M3DB clusters (#2187)

* added new remote write params

* added link to prom params

* adding section for multiple m3db clusters

* first round of edits made

* small edits and additions

* formatting changes

* final changes

* nit

docs/operational_guide/multiple_m3db_clusters.md

@@ -0,0 +1,93 @@
+## Write to multiple M3DB clusters via m3coordinator
+
+### Overview:
+
+Default M3 architecture has the m3coordinator writing to and aggregating metrics from a single M3DB cluster. To map a single coordinator to more than one M3DB cluster, follow the below instructions. 
+
+Use case(s):
+- Sending metrics to different namespaces for different retention periods, etc.
+
+### Instructions: 
+
+1. Add `clusterManagement` to config file to add multiple M3BD clusters to m3coordinator:
+
+Example config file with `clusterManagement` (see end of the config):
+
+```bash
+clusters:
+  # Should match the namespace(s) set up in the DB nodes
+  - namespaces:
+       - namespace: 21d
+         retention: 504h
+         type: unaggregated
+     client:
+       config:
+         service:
+           env: default_env
+           zone: embedded
+           service: m3db
+           cacheDir: /data/m3kv_default
+           etcdClusters:
+             - zone: embedded
+               endpoints:
+              - <ETCD_IP_1>
+              - <ETCD_IP_2>
+              - <ETCD_IP_3>
+       writeRetry:
+         initialBackoff: 500ms
+         backoffFactor: 3
+         maxRetries: 2
+         jitter: true
+       fetchRetry:
+         initialBackoff: 500ms
+         backoffFactor: 2
+         maxRetries: 3
+         jitter: true
+   - namespaces:
+       - namespace: 90d
+         retention: 2160h
+         type: aggregated
+         resolution: 10m
+       - namespace: 500d
+         retention: 12000h
+         type: aggregated
+         resolution: 1h
+     client:
+       config:
+         service:
+           env: lts_env
+           zone: embedded
+           service: m3db
+           cacheDir: /data/m3kv_lts
+           etcdClusters:
+             - zone: embedded
+               endpoints:
+                 - <ETCD_IP_1>
+                 - <ETCD_IP_2>
+                 - <ETCD_IP_3>
+       writeRetry:
+         initialBackoff: 500ms
+         backoffFactor: 3
+         maxRetries: 2
+         jitter: true
+       fetchRetry:
+         initialBackoff: 500ms
+         backoffFactor: 2
+         maxRetries: 3
+         jitter: true
+tagOptions:
+  idScheme: quoted
+clusterManagement:
+  etcd:
+    env: default_env
+    zone: embedded
+    service: m3db
+    cacheDir: /data/m3kv_default
+    etcdClusters:
+      - zone: embedded
+        endpoints:
+          - <ETCD_IP_1>
+          - <ETCD_IP_2>
+          - <ETCD_IP_3>
+```
+2. Use the `Cluster-Environment-Name` header for any API requests to the m3coordinator.