WIP: Add changelog for 1.5.0

[dnephin]

DON'T MERGE

Opening a PR to make sure I haven't missed anything, and as an FYI so we don't duplicate the effort.

[thaJeztah]

To prevent these links to break in future, consider linking to a specific commit, or to the document in the release branch/tag

[dnephin]

Normally I'd like to link to a tag, but we don't create the tag until after the official release, so anyone reading these as part of an rc won't get a correct link.

I also can't link to a branch because we'll drop the bump-1.5.0 branch after release.

I guess that only leaves a commit, which is an ugly URL, but not sure what else to do

[thaJeztah]

Good point :/

Perhaps leave it like this during the RC period, and update together with the bump?
Is there a tag created for the RC's?

[bfirsh]

It might be worth sticking this at the bottom, clarifying that it's an experimental feature - it might confuse users if they think this is the default. (How does this work? Do I have to change what I'm doing? etc)

[dnephin]

Updated based on feedback