Add a change log

[idg10]

Although GitHub automatically generates a list of changes for us when we create a release, there's a problem with this: it doesn't capture any context around why particular changes went into a particular release.

For anyone picking up maintenance of this project (e.g., me at the start of this year) the absence of this kind of background can make some decisions hard to comprehend.

So starting with V6, we want to try to capture a release history that describes the goals of any particular release, to complement the automatically generate list of specific technical changes.