Adding dev sprints to schedule page.

Please confirm that the info I've added is correct -- I don't actually know
for certain that those are the dates! I'm happy to correct it if it's inaccurate.

To alleviate some confusion that I've seen out there about why the schedule
page only lists the 9th-11th when the front page says the conference runs
from the 9th to the 13th, I've added some of the text from last year (from
PyCon US originally, I think?) to the stub page for the conference schedule.

In the longer term, this info should likely move to a more permanent home,
like a dedicated "sprints" page -- but the dates should still be on the
schedule along with some basic info.