Move changelog information into separate file

[ron-at-swgy]

This commit moves the changelog information from the README.md to a separate CHANGELOG.md file. This is a common practice:

1. pry - https://github.com/pry/pry/blob/master/CHANGELOG.md
2. nokogiri - https://github.com/sparklemotion/nokogiri/blob/main/CHANGELOG.md
3. colorize - https://github.com/fazibear/colorize/blob/master/CHANGELOG.md
4. rubocop - https://github.com/rubocop/rubocop/blob/master/CHANGELOG.md