config: Link platform:"..." JSON tags with protocol slugs

So that the semantics are clear.  The platform/protocol disconnect is
unfortunate.  "Protocol" was chosen in de3f1af (Remove language
around Solaris being optional as it is covered in compliance language,
2016-08-17, opencontainers#527) because we may have compliance subsets that aren't
linked to platforms [2].  I'd be open to renaming the JSON tag from
platform:"..." -> protocol:"...", but that's probably more change than
it's worth.

[1]: opencontainers#527 (comment)

Signed-off-by: W. Trevor King <wking@tremily.us>

config.md

@@ -2,6 +2,7 @@
 
 The container's top-level directory MUST contain a configuration file called `config.json`.
 The canonical schema is defined in this document, but there is a JSON Schema in [`schema/config-schema.json`](schema/config-schema.json) and Go bindings in [`specs-go/config.go`](specs-go/config.go).
+For properties that are only defined for some [protocols](README.md#compliance), the Go property has a `platform` tag listing those protocols (e.g. `platform:"linux,solaris"`).
 
 The configuration file contains metadata necessary to implement standard operations against the container.
 This includes the process to run, environment variables to inject, sandboxing features to use, etc.