Document changes for release 0.4

Fixes: #60

CHANGES.md

@@ -0,0 +1,12 @@
+# Version 0.4.0
+
+* Several enumerations are now non_exhaustive for future extensions.
+  These are `Tag`, `Type`, `Value`, `PhotometricInterpretation`,
+  `CompressionMethod`, `Predictor`.
+* Enums gained a dedicated method to convert to their TIFF variant value with
+  the specified type. Performing these conversions by casting the discriminant
+  with `as` is not guaranteed to be stable, except where documented explicitly.
+* Removed the num-derive and num dependencies.
+* Added support for decoding `deflate` compressed images.
+* Make the decoder `Limits` customizable by exposing members.
+* Fixed multi-page TIFF encoding writing incorrect offsets.