Add README.md blurb or create RELEASES.md to explain process

[davetsay]

Included a blurb that describes,

While the versions of the Releases should match those of the MGSS System tested versions, there is no guarantee. Artifacts in the Releases are available for all, at self risk. For MGSS System tested versions of Open MCT, contact an AMMOS representative at https://ammos.nasa.gov/contact.