Add DDraceNetwork

The game is a Teeworlds modification that outgrown the original game.
- It has more devs and more players
- It has own Steam page
- It provides a different gameplay.

entries/ddnet.md

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+# DDraceNetwork
+
+- Home: https://ddnet.tw/
+- State: beta
+- Download: https://ddnet.tw/downloads/
+- Platform: Windows, Linux, macOS
+- Keyword: platform, 2D, content open, online, multiplayer co-op
+- Code repository: https://github.com/ddnet/ddnet
+- Code language: C, C++
+- Code license: Custom (free and open source)
+- Code dependency: SDL
+- Assets license: CC-BY-SA-3.0
+
+Cartoon style 2D multiplayer platform game started out as a modification of Teeworlds with a unique cooperative gameplay.
+Work together with up to 64 people and push your way through maps.
+
+## Building