README.md: add "Boxer" to the project list

Author: stephengold

README.md

@@ -27,9 +27,10 @@ The engine is used by several commercial game studios and computer-science cours
  - [Cubic Nightmare](https://jaredbgreat.itch.io/cubic-nightmare)
  - [Chatter Games](https://chatter-games.com)
  - [Exotic Matter](https://exoticmatter.io)
- - [Demon Lord](https://play.google.com/store/apps/details?id=com.dreiInitiative.demonLord&pli=1)
+ - [Demon Lord (on Google Play)](https://play.google.com/store/apps/details?id=com.dreiInitiative.demonLord&pli=1)
  - [Wild Magic](http://wildmagicgame.ru/)
  - [Marvelous Marbles (on Steam)](https://store.steampowered.com/app/2244540/Marvelous_Marbles/)
+ - [Boxer (on Google Play)](https://play.google.com/store/apps/details?id=com.tharg.boxer)
 
 ## Getting started