Add a note on new msub semantics (#106)

Author: DJMcNab

CHANGELOG.md

@@ -42,6 +42,9 @@ This release has an [MSRV][] of 1.86.
 - Breaking: `a.madd(b, c)` and `a.msub(b, c)` now correspond to `a *
   b + c` and `a * b - c` for consistency with `mul_add` in
   std. ([#88][] by [@Ralith][])
+  Previously, `madd` was `a + b * c`, and `msub` was `a - b * c`.
+  Therefore, if you previously had `a.madd(b, c)`, that's now written as `b.madd(c, a)`.
+  And if you had `a.msub(b, c)`, that's now written `b.madd(-c, a)`.
 - Constructors for static SIMD levels are now `const` ([#93][] by [@Ralith][])
 
 ## [0.2.0][] (2025-08-26)