doc: updated credits and changelog.

doc/changelog.dox

@@ -95,6 +95,8 @@ See also:
     @ref Math::DualQuaternion::transformVectorNormalized() delegating to
     respective APIs in the underlying @ref Complex / @ref Quaternion to make
     the transformation API more consistent with @ref Matrix3 / @ref Matrix4
+-   Added @ref Math::reflect() and @ref Math::refract() (see
+    [mosra/magnum#420](https://github.com/mosra/magnum/pull/420))
 
 @subsubsection changelog-latest-new-platform Platform libraries
 

doc/credits.dox

@@ -161,6 +161,8 @@ Are the below lists missing your name or something's wrong?
     --- initial macOS port, various other improvements
 -   **Nathan Ollerenshaw** ([\@matjam](https://github.com/matjam)) --- Ubuntu
     packages in a PPA repository
+-   **Nghia Truong** ([\@ttnghia](https://github.com/ttnghia) ---@ref Math
+    library additions
 -   **Nick Skelsey** ([\@NSkelsey](https://github.com/NSkelsey)) ---
     documentation copy-editing
 -   **Nicholas "LB" Branden** ([\@LB--](https://github.com/LB--)) --- warning