Marks that gem is no longer actively maintained, invites new maintainer or migration option.

[Schwad]

Ernie mentioned in mid 2016 that he was happy to put a reference to baby_squeel in the project if the maintainers were no longer willing to maintain this project https://github.com/activerecord-hackery/squeel/issues/412. It's been three years and no new commits. I think this will be helpful to invite more maintainers as well as give information for those upgrading past 4.

Thank you so much to Ernie and the team for the wonderful work on this project over the years! :)