Update vers-spec root files copied from purl-spec

[mjherzog]

The task is to updated purl-spec files that should have vers-spec data instead of purl-spec data including:

• README.rst
• ADOPTERS.rst
• LICENSE.rst - the last line in the file should refer to ttps://github.com/package-url/vers-spec instead of ttps://github.com/package-url/purl-spec. There may also be a license change if we decide to change the purl-spec license.
  We also want to convert these files to markdown format.