Updated void file from Zepheira

From @erimille , on issue #34:

This VOID file

* simplifies the description to focus on the more common best practices for constructing VOIDs.
* Removes provenance and updates details for the time begin (there are several ways this can be added and we can weave these in as needed moving forward)
* License has been defined as CC0 per F2F discussion
* Owner / Creator / Publisher defined as US NLM (useful for discovery)
* Added "Linked Data" to description (as that is anticipated useful from a discovery perspective of aggregate VOID files)
Add Medical Subject Headings