create README.md

- [ ] 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