Document --constantLabels

This functionality was added in c225a1e
but not documented.

Author: Phil Frost

README.md

@@ -65,6 +65,9 @@ Package vendoring is handled with [`govendor`](https://github.com/kardianos/gove
   Set the log output target and format. e.g. `logger:syslog?appname=bob&local=7` or `logger:stdout?json=true`
   Defaults to `logger:stderr`.
 
+* `constantLabels`
+  Labels to set in all metrics. A list of `label=value` pairs, separated by commas.
+
 ### Environment Variables
 
 The following environment variables configure the exporter:
@@ -101,6 +104,9 @@ The following environment variables configure the exporter:
 * `PG_EXPORTER_EXTEND_QUERY_PATH`
   Path to a YAML file containing custom queries to run. Check out [`queries.yaml`](queries.yaml)
   for examples of the format.
+
+* `PG_EXPORTER_CONTANT_LABELS`
+  Labels to set in all metrics. A list of `label=value` pairs, separated by commas.
 
 Settings set by environment variables starting with `PG_` will be overwritten by the corresponding CLI flag if given.