Add "sh v2 no longer returns RunningCommand by default" to CHANGELOG

[Jacob Hayes]

Hi, thanks for the great package! I updated to v2 recently and was confused by a handful of str object has no attribute .stderr sorts of messages. I didn't see anything in the changelog that seemed relevant until I came across MIGRATING (so thanks for that!). The RunningCommand->str output seems like a pretty big breaking change, so might be nice to add to the CHANGELOG. Cheers!

[amoffatgmi]

Thanks, for some reason I missed that one