Create README.md for p5.js mode

Author: stephanmax

p5js/README.md

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+# p5.js Mode
+
+## Installation
+
+Currently, installing by double-clicking the released `.pdex` file is not working. (Tested with Processing 4.4.7 on MacOS; error says `Could not find mode in the downloaded file.`) But you can rename `p5js.pdex` to `p5js.zip` and unpack that archive into the Processing sketchbook folder; `~/Documents/Processing/modes/p5js` under MacOS. Next time you start the PDE, you have a new p5.js mode in the dropdown.
+
+The console of the PDE is not very vocal about what is happening behind the scenes of the p5.js mode, at the moment. That’s why I suggest to start the PDE via the command line with `DEBUG=true /Applications/Processing.app/Contents/MacOS/Processing` to get the full debug logs. Once the console says `Done in … with pnpm` you can start running your p5.js sketches inside Processing.