README.md: wrote about warnings

Author: hymkor

README.md

@@ -32,7 +32,12 @@ Oracle
 
     $ sqlbless oracle oracle://USERNAME:PASSWORD@HOSTNAME:PORT/SERVICE
 
+- On error, your transaction is not rolled back.
+
 PostgreSQL
 ----------
 
     $ sqlbless postgres "host=127.0.0.1 port=5555 user=USERNAME password=PASSWORD dbname=DBNAME sslmode=disable"
+
+- Autocommit is disabled.  With INSERT, UPDATE, or DELETE, a transaction starts.
+- On error, the transaction is rolled back automatically because it aborted.