[PR] Introduce a CHANGELOG.md document to the project

I've attempted to fill out the 1.2.0 changelog a bit as we'll be
shipping that later in the week. 1.0.0 was our first official
tagged release, so we don't have versioned data before that.

Leaving 1.1.0 blank in the hopes we add some nice stuff there at
some point.

See #144