[pica-etherpad-db] Add CHANGELOG.md

pica-etherpad-db/CHANGELOG.md

+
+## mysql8.picapatch1
+
+* Limit the [mysql binary log](https://dev.mysql.com/doc/refman/8.0/en/binary-log.html) to 30 hours and some other custom configs in [config/binary_log.cnf](config/binary_log.cnf)
+* Remove unnecessary tools (mainly, libsqlite3-0 and gpg)
+
+## Image size and vulnerabilities
+
+* `pica-etherpad-db:mysql8.picapatch1`: 548MB, [54 vulnerabilities](https://gitlab.utc.fr/picasoft/projets/dockerfiles/-/jobs/886295)
+* Base image: `mysql:8.0.19` (2020-03-31): 547MB, [65 vulnerabilities](https://gitlab.utc.fr/picasoft/projets/dockerfiles/-/jobs/884250)