changelog: add new color-parameter constructors

Author: hollasch

CHANGELOG.md

@@ -3,6 +3,12 @@ Change Log -- Ray Tracing in One Weekend
 
 # v3.2.0 (in progress)
 
+### Common
+  - New: Added alternative constructors that take color arguments in addition to the constructors
+    that take `shared_ptr<texture>` arguments, simplifying calling code. This applies to the
+    following classes: `checker_texture`, `constant_medium`, `diffuse_light`, and `lambertian`.
+    (#516)
+
 
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 # v3.1.2 (in progress)