Add xd-plugin-helper

[svschannak]

I would like to add my small helper library, that i will extend in the next weeks and where I love to integrate helpers from other devs too. For now it has a helper to add error hints to nodes on an artboard. It has MIT-License and is available here: https://github.com/svschannak/xd-plugin-helper

[kerrishotts]

Thanks for submitting a PR -- I've merged it in! :-)