#294 update on-line docs to show how to use chessx with external engine

[David Lewis]

I had installed chessx but couldn't get the internal stockfish to be used; nor could I use the Stockfish.app from the Mac App Store (current Big Sur). I am told chessx does not include stockfish anymore. (I have chessx 1.5.6.)

Here is what I did to get external engine usable: 1) installed command-line Stockfish 13 via homebrew for Mac; 2) After clicking menu items "chessx / Preferences", the pop-up dialog does not let me type. Work-around was to click "Add", select any binary at all in my Applications/ folder, and then instead type-in to the fields the Name and Command (as shown), which appears to work so far (I also entered the Directory as shown but am not sure it is correct or being used).