FR: Keep a Changelog

[GitMensch]

It seems that a lot of good things happen here - but I can't easily find out about those because the commit logs are all in Japanese.
I see that an "old style" Changelog in every folder may match this project, but a single one which is added to with each commit would be very helpful. For some motivations and suggestions concerning the format see https://keepachangelog.com/ (or https://keepachangelog.com/ja/)

I think a CHANGELOG.md or would be very helpful in general and also can be linked in the README.md.
For CHANGELOG_JP.md git log (or the online version) can be used.

[GitMensch]

@yutaro-sakamoto: I still think that's both reasonable and important. If you don't think so feel free to "close as not planned".

[yutaro-sakamoto]

I added CHANGELOG.md.
Thank you for your suggestion.