Add note to blog guide about generating responsive image code

For a recent post @grigs helped me figure out how to get Wordpress to autogenerate responsive image `srcset` and `sizes` stuff for a featured image. I wrote up a quick guide in Slack. I don't have time to add this to the guide now, but we should when we have a chance:

https://cloudfour.slack.com/archives/C0L9UQCTU/p1615496409011100