Add info about installing sample data in drupal 8

I know you're in the middle of moving docs around but since this repo is already on labs I figured it's ok to put this up? If this makes it complicated I'm happy to close and resubmit later.

IMHO, the content does belong in here.

I was told we were ready-to-go with directing changes to this repo, and (as a case-in-point) merging !10 (merged) led to quick publication. :)

Aside: I think the preceding text (right above this change) should be reworked a bit to make the section flow better, but that's sort of a pre-existing issue, and I wouldn't want that to cause merge-conflicts. Better in than out.

merged

mentioned in commit a8ceb38b