Missing release notes entry for --upgrade-strategy

[ncoghlan]

I was looking to see when the new --upgrade-strategy option was added (https://github.com/kennethreitz/pipenv/issues/418#issuecomment-310875287), but it doesn't currently seem to be mentioned in the release notes.

While I did figure it out by finding the relevant PR, it would be helpful to backfill an entry into the 9.0.0 release notes.

I'd be happy to submit a PR for that, but I'd need some pointers on how best to retrofit additional news entries for old releases.

[pradyunsg]

This can be closed now that #4542 has merged; also this is a dup of #4418.

[ncoghlan]

Thanks!