Merge pull request #50 from openzim/breaking_changes

Add precision about breaking changes

Author: benoit74

docs/CHANGELOG.md

@@ -1,3 +1,5 @@
 Mandatory for a versioned project. Must follow [Keep A Changelog](https://keepachangelog.com/en/1.1.0/)'s format.
 
-Ideally, entries in response to Github issues should mention them.
+Ideally, entries in response to Github issues should mention them.
+
+All breaking changes must be listed under a single `### Breaking Changes` section per release (instead of prefixing them in `### Changed` or `### Fixed`. And this section must be the first section of the (major obviously) release.