Fix docs to include where to put files on Mac

[mclate]

I've been struggling to make it work until i discovered (completely by chance) this comment: #32 (comment)

Please add the info that one has to extract the archive into ~/Documents/Arduino/tools on mac in order to have it running.

Thank you.

[EgHubs]

Thank you

[IanBUK]

This isn't working for me - I still can't see the extension.

[zhekch]

This isn't working for me - I still can't see the extension.

are you using arduino 2.x.x? If yes, the extension dont work in arduino 2.x.x for some reason. Use arduino 1.8