Add changelog

[chevinbrown]

Can we please start using a changelog?

Working through commits is a difficult way to review changes. Having a changelog with a concise list of updates will be very helpful in updating the gem.

[mrjamesriley]

Would certainly appreciate there being a changelog here. Especially with the release of v1 - where there's no indication of what's changed with the major version.

[tisba]

Now with 2.0.0 being released, it would be very much appreciated if we could get a changelog. Most importantly: Are there breaking changes between 1.0.0 and 2.0.0?

[brendon]

Just commenting on what I could see in the commits between version 1.0.0 and 2.0.1, there doesn't seem to be any actual functionality change, just changes to the test matrix, adding testing against things like Rails 6 and newer Ruby versions plus removing support for older Ruby and Rails versions. Still, a Changelog would be useful :)

[brendon]

Back again :D From 2.0.1 to 2.1.0 there also don't appear to be any material changes to functionality and just one bug fix around mailer generation.