docs: added test reference

README.md

@@ -11,6 +11,7 @@ A Game Boy emulator that is written in Rust 🦀.
 * Web and SDL front-ends
 * Support for multiple MBCs: MBC1, MBC2, MBC3, and MBC5
 * Variable CPU clock speed
+* Passes [dmg-acid2](https://github.com/mattcurrie/dmg-acid2) PPU tests
 
 For the Web front-end...