Update readme to include Postgres 14 support

It looks like postgres 14.1 was added to CI here:

prometheus-community@fcb2535

See also: prometheus-community#651 (comment)

Signed-off-by: Austin Godber <godber@uberhip.com>

README.md

@@ -7,7 +7,7 @@
 
 Prometheus exporter for PostgreSQL server metrics.
 
-CI Tested PostgreSQL versions: `9.4`, `9.5`, `9.6`, `10`, `11`, `12`, `13`
+CI Tested PostgreSQL versions: `9.4`, `9.5`, `9.6`, `10`, `11`, `12`, `13`, `14`
 
 ## Quick Start
 This package is available for Docker: