Document INTERFACE_TYPE as the emulator selector

[mikelangmayr]

• corrects two mistakes I introduced in Correct the build and run instructions in the README #20: it said -DINTERFACE_TYPE=AstroCam is "not an option this project defines", and that the emulator "is built only for -DCONTROLLER=archon"
• INTERFACE_TYPE is defined, in the top-level CMakeLists, defaulting to Archon when not set, and it is what the emulator's CMakeLists branches on
• it is independent of CONTROLLER: verified that -DCONTROLLER=archon -DINTERFACE_TYPE=AstroCam reports "emulator not implemented for AstroCam" and produces no emulator target, so the emulator follows INTERFACE_TYPE and not the controller
• the old README's ARC column was still wrong, but because INTERFACE_TYPE does not select the controller, not because it does not exist