MySQL 5.6 default config creates bug

Added some specification for the MySQL config in the README.

As of MySQL 5.6 config is more strict. The STRICT_TRANS_TABLES is now a default. This creates sql errors when the value inserted/updated is empty or invalid and the column is defined as NOT NULL with no Default value.

In my opinion, changing the configuration is not a definitive solution. A more suitable solution would be to change the database schema to define the optionals data as nullable.

- Example: 
In table address: fields address2 and address3 are Not Null and no default value. 
When trying to save a client with no data in those two fields, an error occurs.
Filling the two fields solves the problem.