Document provenance of Penner easing equations in Tween

As noted in #21600.

COPYRIGHT.txt

@@ -96,6 +96,11 @@ Copyright: 1997-2017, Sam Lantinga
   2014-2018, Godot Engine contributors.
 License: Expat and Zlib
 
+Files: ./scene/animation/tween_interpolaters.cpp
+Comment: Penner Easing
+Copyright: 2001, Robert Penner
+License: BSD-3-clause
+
 Files: ./servers/physics/gjk_epa.cpp
  ./servers/physics/joints/generic_6dof_joint_sw.cpp
  ./servers/physics/joints/generic_6dof_joint_sw.h

scene/animation/tween_interpolaters.cpp

@@ -28,6 +28,12 @@
 /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
 /*************************************************************************/
 
+/**
+ *  Adapted from Penner Easing equations' C++ port.
+ *  Source: https://github.com/jesusgollonet/ofpennereasing
+ *  License: BSD-3-clause
+ */
+
 #include "tween.h"
 
 const real_t pi = 3.1415926535898;