Add readme for rethinkdb input (#6211)

plugins/inputs/rethinkdb/README.md

@@ -0,0 +1,61 @@
+# RethinkDB Input
+
+Collect metrics from [RethinkDB](https://www.rethinkdb.com/).
+
+### Configuration
+
+This section contains the default TOML to configure the plugin.  You can
+generate it using `telegraf --usage rethinkdb`.
+
+```toml
+[[inputs.rethinkdb]]
+  ## An array of URI to gather stats about. Specify an ip or hostname
+  ## with optional port add password. ie,
+  ##   rethinkdb://user:auth_key@10.10.3.30:28105,
+  ##   rethinkdb://10.10.3.33:18832,
+  ##   10.0.0.1:10000, etc.
+  servers = ["127.0.0.1:28015"]
+  
+  ## If you use actual rethinkdb of > 2.3.0 with username/password authorization,
+  ## protocol have to be named "rethinkdb2" - it will use 1_0 H.
+  # servers = ["rethinkdb2://username:password@127.0.0.1:28015"]
+  
+  ## If you use older versions of rethinkdb (<2.2) with auth_key, protocol
+  ## have to be named "rethinkdb".
+  # servers = ["rethinkdb://username:auth_key@127.0.0.1:28015"]
+```
+
+### Metrics
+
+- rethinkdb
+  - tags:
+    - type
+    - ns
+    - rethinkdb_host
+    - rethinkdb_hostname
+  - fields:
+    - cache_bytes_in_use (integer, bytes)
+    - disk_read_bytes_per_sec (integer, reads)
+    - disk_read_bytes_total (integer, bytes)
+    - disk_written_bytes_per_sec (integer, bytes)
+    - disk_written_bytes_total (integer, bytes)
+    - disk_usage_data_bytes (integer, bytes)
+    - disk_usage_garbage_bytes (integer, bytes)
+    - disk_usage_metadata_bytes (integer, bytes)
+    - disk_usage_preallocated_bytes (integer, bytes)
+  
+- rethinkdb_engine
+  - tags:
+    - type
+    - ns
+    - rethinkdb_host
+    - rethinkdb_hostname
+  - fields:
+    - active_clients (integer, clients)
+    - clients (integer, clients)
+    - queries_per_sec (integer, queries)
+    - total_queries (integer, queries)
+    - read_docs_per_sec (integer, reads)
+    - total_reads (integer, reads)
+    - written_docs_per_sec (integer, writes)
+    - total_writes (integer, writes)