add contributor guide and licence

[researcherben]

Something akin to https://github.com/sstsimulator/sst-core/blob/master/CONTRIBUTING.md
to help folks who want to add content.

What is the voice used throughout the content? (First person, third person).

How to indicate which version of SST is being referred to? Or is the assumption that only the current release is described?

[gvoskuilen]

Thanks @researcherben for the suggestion. I will work on getting something into the next round of doc updates.