Add info to postgresql connector

Author: psainics

postgresql-plugin/docs/PostgreSQL-connector.md

@@ -26,6 +26,8 @@ authentication. Optional for databases that do not require authentication.
 - TRANSACTION_READ_COMMITTED: No dirty reads. Non-repeatable reads and phantom reads are possible.
 - TRANSACTION_SERIALIZABLE: No dirty reads. Non-repeatable and phantom reads are prevented.
 - TRANSACTION_REPEATABLE_READ: No dirty reads. Prevents non-repeatable reads, but phantom reads are still possible.
+- PostgreSQL's `TRANSACTION_READ_UNCOMMITTED` mode behaves like `TRANSACTION_READ_COMMITTED`, hence it's not included as a separate
+  option, for more info see: https://www.postgresql.org/docs/current/transaction-iso.html#TRANSACTION-ISO
 
 **Database:** The name of the database to connect to.