create README.md

[Tyler-IN]

• [ ] prerequisites
• [ ] install instructions
• [ ] explain benefits of .NET 6
• [ ] explain AppContainer isolation security
• [ ] explain creating a mod to change content definitions
• [ ] explain creating a mod to change bundled assets
• [ ] explain creating a mod to run some code
• [ ] explain creating a mod to apply a harmony patch
• [ ] explain troubleshooting a harmony / code mod