add contributing section

Signed-off-by: Jonathan Alvarez <[email protected]>

README.md

@@ -7,3 +7,11 @@ It contains:
 - `example` directory: exemplary VC for each assertion type
 - `packages/schema` directory: JSONSchema for each assertion type
 - `packages/validator` directory: Utility to validate schemas from a VC string.
+
+## Contributing
+
+1. Locate the schema you want to update on `packages/schema`. You can tell it from the VC's `credentialSchema.id` value.
+1. Follow the versioning described in `packages/schema`'s README to name the new version.
+1. The README describes how to generate the schemas from its model too.
+1. Update the CHANGELOG.
+1. Publish the pull request.