Added sketchpack.json

Hey, I realised you were trying to log into sketch.in and you got an error. Sorry for that. Its fixed now. Please feel free to submit your plugin now. I am sure it will work this time.
I have gone on and created a sketchpack.json file for you. This file is required to be present in your repo for you to be able to add your plugin to sketchpack.in,  you can edit it as you see fit.
Looking forward to seeing your plugin on sketchpack!
Thanks.