Add "compliance achieved" date to browser support table.

[hwine]

As a person who is not totally conversant with all the various browser release dates and versions, it would be helpful if the "supported version table" included the first date all listed browsers met that level. [That's worded awkwardly -- I mean one date per compliance level -- month/year.]

I believe this addition would make potential impact of choosing a more modern level easier to reason about.