Changelog missing for 1.5

[tony]

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Related #133

Based on the README and the issues, it's not clearly obvious if 1.11 support has been added since changelog isn't updated

Also it's not possible to know what changed when moving from 1.4 -> 1.5

(of course obviously someone could read through git history... but at that rate, people may begin coding their own category solution even though django-categories could help them :smile:)