Split parser/serializer docs (#4690)

plugins/parsers/value/README.md

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+# Value
+
+The "value" data format translates single values into Telegraf metrics. This
+is done by assigning a measurement name and setting a single field ("value")
+as the parsed metric.
+
+### Configuration
+
+You **must** tell Telegraf what type of metric to collect by using the
+`data_type` configuration option. Available options are:
+
+1. integer
+2. float or long
+3. string
+4. boolean
+
+**Note:** It is also recommended that you set `name_override` to a measurement
+name that makes sense for your metric, otherwise it will just be set to the
+name of the plugin.
+
+```toml
+[[inputs.exec]]
+  ## Commands array
+  commands = ["cat /proc/sys/kernel/random/entropy_avail"]
+
+  ## override the default metric name of "exec"
+  name_override = "entropy_available"
+
+  ## 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 = "value"
+  data_type = "integer" # required
+```
+