Update the CHANGELOG for 8.20.

CHANGELOG

@@ -1,3 +1,34 @@
+Quake II 8.10 to 8.20:
+- This release marks Quake IIs 25th anniversary. Happy Birthday!
+- Various improvements to the menu. This includes updates to the menu
+  framework itself, to the game controller menu, the savegame menu and
+  the video menus. (by apartfromtime, protocultor, David Carlier and
+  0lvin)
+- A lot of fixes and improvements to the controller support. Support for
+  gyro aiming, more precise stick handling and rumble improvements. (by
+  protocultor)
+- Implement faster weapon switching with the new 'cycleweap' command.
+  (by protocultor).
+- Fixes pusher delta yaw manipulation. This fixes the infamous bug were
+  a player standing on a blocked elevator gets turned around (by
+  skuller).
+- Add a command 'spawnentity' to spawn entities at the given
+  coordinates. Can be used for debugging. (by 0lvin)
+- Ported monster footstep sounds from SkacikPLs Q2RTX fork. Needs a pak
+  file with assets to work, see the documentation for details. (by
+  0lvin)
+- Ported reverb effects from SkacikPLs Q2RTX fork. (by 0lvin)
+- Fix several coop related bugs with the powercubes. (by BjossiAlfreds)
+- A way better fix for dead bodys obstructing elevators or falling
+  through the worldmodel. (by BjossiAlfreds)
+- Fix items already in water playing a splash sound at level start. (by
+  BjossiAlfreds)
+- Pause the game when getting minimized or hidden. (by David Carlier)
+- Fix Bugs related to 'ogg_ignoretrack0'. (by BjossiAlfreds)
+- Share model loading code between renderers. (by 0lvin)
+- Restore stair step smoothing for step sizes other than 16.
+- Fix playback of long audio samples (>10 seconds) through OpenAL.
+
 Quake II 8.01 to 8.10:
 - The OpenGL 3.2 renderer now supports the classic underwater effect.
   Until now it was only available in the Software renderer.