Clarify that this repo is OLD and no longer used

[kwalrath]

(However, we're leaving it around for reference. For now, at least. The README is particularly helpful.)

Link to the new repos:

https://github.com/dart-lang/site-www https://github.com/dart-lang/site-webdev https://github.com/dart-lang/site-events

[kwalrath]

I've updated the README.

I need an owner (could be me, if you grant me access) to change the description on the front page.

[kevmoo]

Good enough?

[kwalrath]

LGTM. Thanks!