Add ASN.1 schema export from static serialization templates.

Introduce gmser_schema_export to emit portable ASN.1 type definitions
from tag/version/template triples (RLP remains the wire format). Export
tag/1 and rev_tag/1 for discovery and naming, with docs and eunit coverage.

README.md

@@ -4,6 +4,10 @@ Serialization helpers for the Gajumaru.
 
 For an overview of the static serializer, see [this document](doc/static.md).
 
+To export static templates as portable ASN.1 type definitions (for codegen
+in other languages; wire format remains RLP), see
+[doc/schema_export.md](doc/schema_export.md) and module `gmser_schema_export`.
+
 ## Build
 
     $ rebar3 compile