Update changelog

CHANGELOG.md

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 ### Added
 - `avifPixelFormatToString()` convenience function for debugging/printing
 - `avifenc` and `avifdec` "apps" which show basic bidirectional conversion to y4m
+- Remove all calls to `convertXYZToXYY()` as they were all unnecessary
+- Make calling `avifImageYUVToRGB()` upon reading an avif optional
 
 ## [0.1.3] - 2019-04-23
 ### Changed