[Suggestion] Add some documentation about the individual emulators

Hi, perhaps it wouldn't be a bad idea to include some minimal documentation for the different emulation cores. This could cut down on people submitting issues about for simple questions that they could look up themselves.

Or at the very least, include links to the official site for each core. This could take the guesswork out of trying to find out which core is being used for a particular emulator and people could look up stuff if they have issues or want to customize the emulator.

No file, test, or entry point is named. Start by locating the project's documentation and the mapping between emulators and their individual cores. Done means each core has minimal usage information or an official-site link, with enough detail to identify which core an emulator uses.

Written by the indexing model from the issue text.