Add a note on add_assign_mixed to CHANGELOG (arkworks-rs#600)

Author: mmagician

CHANGELOG.md

@@ -71,6 +71,7 @@
 - [\#446](https://github.com/arkworks-rs/algebra/pull/446) (`ark-ff`) Add `CyclotomicMultSubgroup` trait and implement it for extension fields
 - [\#447](https://github.com/arkworks-rs/algebra/pull/447) (`ark-ec`, `ark-algebra-test-templates`) Rename and refactor group infrastructure, and test infrastructure for fields, groups, and pairings:
     - Create new `Group` trait and move some functionality from `ProjectiveCurve` to it.
+    - Refactor `add_assign_mixed` → `add_assign` that's polymorphic over its RHS.
     - Rename `ProjectiveCurve` to `CurveGroup: Group`.
         - Rename some associated types:
             - `AffineCurve` → `Affine`

ec/src/models/short_weierstrass/group.rs

@@ -330,6 +330,7 @@ impl<P: SWCurveConfig> Neg for Projective<P> {
 }
 
 impl<P: SWCurveConfig, T: Borrow<Affine<P>>> AddAssign<T> for Projective<P> {
+    /// Using http://www.hyperelliptic.org/EFD/g1p/auto-shortw-jacobian-0.html#addition-madd-2007-bl
     fn add_assign(&mut self, other: T) {
         let other = other.borrow();
         if let Some((&other_x, &other_y)) = other.xy() {