Split parser/serializer docs (#4690)

plugins/parsers/nagios/README.md

@@ -0,0 +1,17 @@
+# Nagios
+
+The `nagios` data format parses the output of nagios plugins.
+
+### Configuration
+
+```toml
+[[inputs.exec]]
+  ## Commands array
+  commands = ["/usr/lib/nagios/plugins/check_load -w 5,6,7 -c 7,8,9"]
+
+  ## Data format to consume.
+  ## Each data format has its own unique set of configuration options, read
+  ## more about them here:
+  ##   https://github.com/influxdata/telegraf/blob/master/docs/DATA_FORMATS_INPUT.md
+  data_format = "nagios"
+```