Update README.md (#120)
Update README.md (#120)

- Fixed formatting issues
- Updated build URL (source for url [here](https://forums.spongepowered.org/t/prism-official-thread-v3-0-beta/1137))
- Fixed header image
- Removed donation section as requested

Stonebound/Prism,prism/Prism,prism/Prism,Stonebound/Prism