[circleci] Generate change log for automatically generated release

[stefan-guggisberg]

Is your feature request related to a problem? Please describe.
Follow-up on #306: It would be nice if the change log since the last release (currently a TODO placeholder) would be automatically generated, even if it might need to be manually edited afterwards.

Describe the solution you'd like
The change log could be compiled from the commit messages since the last release, e.g. something like

git log ${prev_release_tag}...master | fgrep \#

Describe alternatives you've considered
semantic-release. seems overkill for the task at hand.

Additional context
#306

[stefan-guggisberg]

implemented in ad215bd