Time based SSG releases

[Watson Yuuma Sato]

Hello folks,

I would like to announce that we are changing SCAP Security Guide releases
to be time based. From now on, every two months there will be a release
coming out.

As last release was on April 29th, next release will happen on June 29th.

Our aim is to release early and often to increase the feedback loop
and also reduce anxiety on when the next release will happen.