Write a styleguide for our CHANGELOG.md.
This is based in part on the C Tor changelog standards and style guide, in part on an old hacking styleguide I found, and in part on past changelogs.
It also includes a template to help start a new changelog.
Please do not merge this without discussion! I suspect that @Diziet and @gabi-250 will have opinions; @arma may also, since he has cared strongly about C Tor changelog formatting in the past.