Add instruction on when to add org label

[pllim]

Motivation: Over at Astropy, we have a link to "packages under review" and rely on the astropy label being applied for this list to be accurate. See astropy/astropy.github.com#595

It would be nice to provide instruction to EiC or Editor on when to add this label.

cc @lwasser @hamogu @dhomeier

[dhomeier]

Will also be needed to identify them for the yaml file feeding the Astropy landing page here (see #268)