Document compatibility for latest Postgres versions (#116)

Author: Charlie Hornsby

README.md

@@ -363,7 +363,7 @@ Compatibility
 
 This module has been tested on:
 
-* **Postgres 9.4, 9.5, 9.6, 10, 11**
+* **Postgres 9.4, 9.5, 9.6, 10, 11, 12, 13**
 
 If you end up needing to change something to get this running on another system, send us the diff and we'll try to work it in!