Update README to confirm iOS 13 support.

Author: pitfield

README.md

@@ -92,7 +92,7 @@ PostgresClientKit is compatible with Linux, macOS, and iOS.  It has been tested
 
 - Ubuntu 18.04 LTS 
 - macOS 10.14
-- iOS 12
+- iOS 12, 13
 - Postgres 10, 11, 12
 
 ## Building