Adding GNU-Style ChangeLog file

Hi,

ChangeLog file is an important file to have and can be easily generated from the git
log using the `gitlog-to-changelog` command from the `gnulib` package.
Options used in that case are:
--format='%s%n%n%b%n' --no-cluster --strip-tab --strip-cherry-pick

In Debian we need to have it separated from the Readme, but if you don't want to bother with that, even if it's easy, I would completely understand.

Best,
Joseph