Document supported spec versions

Fixes #130

README.md

@@ -7,6 +7,9 @@ The Software Package Data Exchange (SPDX) specification is a standard format for
 These tools are published by the SPDX Workgroup
 see [http://spdx.org/](http://spdx.org/)
 
+## Versions Supported
+This utility supports versions 2.0, 2.1, 2.2 and 2.3 of the SPDX specification.
+
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