update README

README.md

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-emu_nes
-=======
+# emu_nes
 A Nintendo Entertainment System (NES) emulator written for the purpose of
 understanding how the NES and similar 8-bit computers of that era worked.
 
 Makes heavy use of https://www.nesdev.org/.
+
+## Current State
+- [x] CPU (2A03/6502)
+- [ ] PPU (Graphics)
+- [ ] APU (Audio)
+
+Currently, running the emulator with nestest.nes results in incorrect
+results in the memory space of the APU registers. This is due to the
+APU not being implemented yet.
+
+Also, the only ROM mapper that is implemented is Mapper 0 to
+run the test ROM, but it also supports many other ROMs listed
+[here](https://nesdir.github.io/mapper0.html) such as Donkey Kong and
+Super Mario Bros.