Add some repository tags

[MrBrixican]

Having relevant tags can help with discoverability.

Some good ones may be:

• game-framework
• game-engine
  • well aware this is a framework, but libgdx, monogame, etc. use it
• game-development
• gamedev
• cross-platform
• open-source
• graphics
• 2d
• c-sharp
• csharp
• dotnet

[NoelFB]

Makes sense! Done!