Update CHANGELOG.md for 4.0 release

We couldn't hope to complete an in-depth curated changelog for such
a massive release, so we simply link to our in-depth release notes :)

Also adds back the 3.3, 3.4, and 3.5 curated changelogs. Those were
released from the 3.x branch so theoretically do not share a common
history with 4.0 (which started development after 3.2), but in practice
users will still perceive 4.0 as the natural upgrade to 3.5.