Add spec start dates

Currently the dates are the last time the specs were updated, which feedback shows makes for a confusing experience. Spec start dates should be added in the format 2012-2015, where 2015 is the end date.

Ideally, the spec start date should also link to the first version of the spec that was implemented. Or an article on the history– I'm not sure which yet. However, we can do that separately.