TBB 3.x's ChangeLog pretends the world started at TBB 3.0a1

Docs/ChangeLog.txt in tbb 3.0a4 has only four entries in it.

If our goal is to replace TBB 2.x, we will at some point want to import its whole changelog.

(A changelog is used a) for learning what changed in the newest version, and b) for answering questions like "when did tbb change feature x?")