Document Go version policy

Signed-off-by: Brandon Mitchell <git@bmitch.net>

Author: sudo-bmitch

HACKING.md

@@ -10,7 +10,7 @@ This spec includes several Go packages, and a command line tool considered to be
 
 Prerequisites:
 
-- Go - current release only, earlier releases are not supported
+- Go - latest version is recommended, see the [go.mod](go.mod) file for the minimum requirement
 - make
 
 The following make targets are relevant for any work involving the Go packages.

go.mod

@@ -1,5 +1,8 @@
 module github.com/opencontainers/image-spec
 
+// The minimum Go release is only incremented when required by a feature.
+// At least 3 Go releases will be supported by the spec.
+// For example, updating this version to 1.19 first requires Go 1.21 to be released.
 go 1.18
 
 require (