How to enable explicit_defaults_for_timestamp?

When I try to start my mySQL server I get message:

[Warning] TIMESTAMP with implicit DEFAULT value is deprecated. Please use --explicit_defaults_for_timestamp server option (see documentation for more details).

I find answer on:
http://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_explicit_defaults_for_timestamp

But how enable it? Where?