Improve README for couchabase input plugin

Proper terminology and case. Exmaples for tags. Example output.

plugins/inputs/couchbase/README.md

 # Telegraf Plugin: Couchbase
 
-### Configuration:
+## Configuration:
 
 ```
 # Read per-node and per-bucket metrics from Couchbase
@@ -19,28 +19,45 @@
 
 ## Measurements:
 
-### Per-node measurements
+### couchbase_node
 
-Meta:
-- units: bytes
-- tags: `cluster`, `hostname`
+Tags:
+- cluster: whatever you called it in `servers` in the configuration, e.g.: `http://couchbase-0.example.com/`
+- hostname: Couchbase's name for the node and port, e.g., `172.16.10.187:8091`
 
-Measurement names:
-- memory_free (example: 23181365248.0)
-- memory_total (example: 64424656896.0)
+Fields:
+- memory_free (unit: bytes, example: 23181365248.0)
+- memory_total (unit: bytes, example: 64424656896.0)
 
-### Per-bucket measurements
+### couchbase_bucket
 
-Meta:
-- units: varies
-- tags: `cluster`, `bucket`
+Tags:
+- cluster: whatever you called it in `servers` in the configuration, e.g.: `http://couchbase-0.example.com/`)
+- bucket: the name of the couchbase bucket, e.g., `blastro-df`
 
-Measurement names:
-- quotaPercentUsed (unit: percent, example: 68.85424936294555)
-- opsPerSec (unit: count, example: 5686.789686789687)
-- diskFetches (unit: count, example: 0.0)
-- itemCount (unit: count, example: 943239752.0)
-- diskUsed (unit: bytes, example: 409178772321.0)
-- dataUsed (unit: bytes, example: 212179309111.0)
-- memUsed (unit: bytes, example: 202156957464.0)
+Fields:
+- quota_percent_used (unit: percent, example: 68.85424936294555)
+- ops_per_sec (unit: count, example: 5686.789686789687)
+- disk_fetches (unit: count, example: 0.0)
+- item_count (unit: count, example: 943239752.0)
+- disk_used (unit: bytes, example: 409178772321.0)
+- data_used (unit: bytes, example: 212179309111.0)
+- mem_used (unit: bytes, example: 202156957464.0)
 
+
+## Example output
+
+```
+$ telegraf -config telegraf.conf -input-filter couchbase -test
+* Plugin: couchbase, Collection 1
+> couchbase_node,cluster=https://couchbase-0.example.com/,hostname=172.16.10.187:8091 memory_free=22927384576,memory_total=64424656896 1458381183695864929
+> couchbase_node,cluster=https://couchbase-0.example.com/,hostname=172.16.10.65:8091 memory_free=23520161792,memory_total=64424656896 1458381183695972112
+> couchbase_node,cluster=https://couchbase-0.example.com/,hostname=172.16.13.105:8091 memory_free=23531704320,memory_total=64424656896 1458381183695995259
+> couchbase_node,cluster=https://couchbase-0.example.com/,hostname=172.16.13.173:8091 memory_free=23628767232,memory_total=64424656896 1458381183696010870
+> couchbase_node,cluster=https://couchbase-0.example.com/,hostname=172.16.15.120:8091 memory_free=23616692224,memory_total=64424656896 1458381183696027406
+> couchbase_node,cluster=https://couchbase-0.example.com/,hostname=172.16.8.127:8091 memory_free=23431770112,memory_total=64424656896 1458381183696041040
+> couchbase_node,cluster=https://couchbase-0.example.com/,hostname=172.16.8.148:8091 memory_free=23811371008,memory_total=64424656896 1458381183696059060
+> couchbase_bucket,bucket=default,cluster=https://couchbase-0.example.com/ data_used=25743360,disk_fetches=0,disk_used=31744886,item_count=0,mem_used=77729224,ops_per_sec=0,quota_percent_used=10.58976636614118 1458381183696210074
+> couchbase_bucket,bucket=demoncat,cluster=https://couchbase-0.example.com/ data_used=38157584951,disk_fetches=0,disk_used=62730302441,item_count=14662532,mem_used=24015304256,ops_per_sec=1207.753207753208,quota_percent_used=79.87855353525707 1458381183696242695
+> couchbase_bucket,bucket=blastro-df,cluster=https://couchbase-0.example.com/ data_used=212552491622,disk_fetches=0,disk_used=413323157621,item_count=944655680,mem_used=202421103760,ops_per_sec=1692.176692176692,quota_percent_used=68.9442170551845 1458381183696272206
+```