Document how to export uf2 from Arduino

[jpmeijers]

Using Arduino to compile your sketch, it's not very clear how to obtain a uf2 to copy to the MSC. What I figured out and what seems to work is:

• Sketch -> Verify/Compile sketch
• Sketch -> Export compiled Binary
• Sketch -> Show Sketch Folder
• Download https://github.com/Microsoft/uf2/blob/master/utils/uf2conv.py into the sketch directory and make it executable
• Open a terminal and run ./uf2conv.py -c SketchName.ino.board_name.bin
• You should now have a flash.uf2 file in the sketch folder which you cna copy-paste to the mass storage drive.

[dhalbert]

Would you like to submit a PR to update README.md to document this? Thanks!

[dhalbert]

This is perhaps better documented in the https://github.com/Microsoft/uf2 repo. EIther it's already there or a PR to that README could be written.