Create a CHANGELOG file for each Spree component

It should help us to have a detailed idea of what changes between
versions. Both community and core developers could add an entry for
every relevant change. e.g. deprecated methods, relevant API
improvements, fixes.

I've been missing a CHANGELOG in Spree for a while. The project is huge and it's hard too keep track of changes only by reading commit messages. Please let me know your thoughts on this.