Write release notes for the curve derivative work (#2064)

Author: alice-i-cecile

release-content/0.16/release-notes/16503_Derivative_access_patterns_for_curves.md

@@ -1,4 +1,34 @@
-<!-- Derivative access patterns for curves -->
-<!-- https://github.com/bevyengine/bevy/pull/16503 -->
+`bevy_math` has collected a sizable collection of curves and methods for working with curves, which are useful for everything from animations to color gradients to gameplay logic.
 
-<!-- TODO -->
+One of the most natural and important things you might want to do with a curve is to inspect its **derivative**:
+the rate at which it's changing.
+You might even be after its **second derivative**: the rate at which the rate of change is changing.
+
+Accessing this information should be easy and error-resistant,
+but we also won't accept any complication or performance penalty in the more common derivative-less use cases.
+
+To satisfy these requirements, our resident mathematicians have built a careful solution involving multiple traits and wrapper types.
+For a full description of the mechanisms and considerations, dive down the rabbit hole into the linked PR description!
+Despite (or because of?) all of the mathematical type wizardry, the actual usage is straightforward:
+
+```rust
+let points = [
+    vec2(-1.0, -20.0),
+    vec2(3.0, 2.0),
+    vec2(5.0, 3.0),
+    vec2(9.0, 8.0),
+];
+
+// A cubic spline curve that goes through `points`.
+let curve = CubicCardinalSpline::new(0.3, points).to_curve().unwrap();
+
+// Calling `with_derivative` causes derivative output to be included in the output of the curve API.
+let curve_with_derivative = curve.with_derivative();
+
+// A `Curve<f32>` that outputs the speed of the original.
+let speed_curve = curve_with_derivative.map(|x| x.derivative.norm());
+```
+
+We've implemented the required traits for most of our native curve types: splines, lines, and all manner of compound curves.
+Curves which accept arbitrary functions are not covered (build your own specialized curve types),
+as Rust does not have a first-class notion of a differentiable function!

release-content/0.16/release-notes/_release-notes.toml

@@ -187,7 +187,7 @@ file_name = "15074_bevy_reflect_Function_Overloading_Generic__Variadic_Functi.md
 # Math
 
 [[release_notes]]
-title = "Derivative access patterns for curves"
+title = "Curve derivatives"
 authors = ["@mweatherley"]
 contributors = ["@alice-i-cecile"]
 prs = [16503]