Mention partner runners in README

There are public partner runners available on GitHub, such as ARM
runners for Windows and Linux. Since we recently added their support, it
worth mentioning them.

Author: ikalnytskyi

README.md

@@ -7,12 +7,13 @@
 This action sets up a PostgreSQL server for the rest of the job. Here are some
 key features:
 
-* Runs on Linux, macOS and Windows action runners.
+* Runs on Linux, macOS and Windows action runners, including [partner][0] runners.
 * PostgreSQL version can be parametrized.
 * Adds PostgreSQL [client applications][1] to `PATH`.
 * Supports SSL on-demand.
 * Easy [to verify][2] that it DOES NOT contain malicious code.
 
+[0]: https://github.com/actions/partner-runner-images/
 [1]: https://www.postgresql.org/docs/current/reference-client.html
 [2]: action.yml