Update MySQL supported versions

Author: alex-kulakov

ReadMe.md

@@ -7,7 +7,7 @@ Supported databases:
 - MS Azure SQL Database
 - Oracle 10g, 11g
 - PostgreSQL 8.3, 8.4, 9.0, 9.1, 9.2, 10, 11
-- MySQL 5.5, 5.6
+- MySQL 5.5, 5.6, 5.7, 8.0
 - Firebird 2.5
 - Sqlite 3