Split parser/serializer docs (#4690)

plugins/parsers/collectd/README.md

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+# Collectd
+
+The collectd format parses the collectd binary network protocol.  Tags are
+created for host, instance, type, and type instance.  All collectd values are
+added as float64 fields.
+
+For more information about the binary network protocol see
+[here](https://collectd.org/wiki/index.php/Binary_protocol).
+
+You can control the cryptographic settings with parser options.  Create an
+authentication file and set `collectd_auth_file` to the path of the file, then
+set the desired security level in `collectd_security_level`.
+
+Additional information including client setup can be found
+[here](https://collectd.org/wiki/index.php/Networking_introduction#Cryptographic_setup).
+
+You can also change the path to the typesdb or add additional typesdb using
+`collectd_typesdb`.
+
+### Configuration
+
+```toml
+[[inputs.file]]
+  files = ["example"]
+
+  ## 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 = "collectd"
+
+  ## Authentication file for cryptographic security levels
+  collectd_auth_file = "/etc/collectd/auth_file"
+  ## One of none (default), sign, or encrypt
+  collectd_security_level = "encrypt"
+  ## Path of to TypesDB specifications
+  collectd_typesdb = ["/usr/share/collectd/types.db"]
+
+  ## Multi-value plugins can be handled two ways.
+  ## "split" will parse and store the multi-value plugin data into separate measurements
+  ## "join" will parse and store the multi-value plugin as a single multi-value measurement.
+  ## "split" is the default behavior for backward compatability with previous versions of influxdb.
+  collectd_parse_multivalue = "split"
+```