Merge pull request opencontainers#500 from wking/document-go-compat

README: Link to specs-go and schema and declare Go-compat policy

Author: Stephen Day

README.md

@@ -9,6 +9,9 @@ The OCI Image Format project creates and maintains the software shipping contain
 
 The specification can be found [here](spec.md).
 
+This repository also provides [Go types](specs-go), [intra-blob validation tooling, and JSON Schema](schema).
+The Go types and validation should be compatible with the current Go release; earlier Go releases are not supported.
+
 Additional documentation about how this group operates:
 
 - [Code of Conduct](https://github.com/opencontainers/tob/blob/d2f9d68c1332870e40693fe077d311e0742bc73d/code-of-conduct.md)