chore: add input format to README

README.adoc

@@ -79,6 +79,17 @@ The TCK will take over from there and run all the tests by hooking in the specif
 In order to pass the AsciiDoc TCK tests, you must create a CLI interface that reads AsciiDoc content from stdin and return an Abstract Semantic Graph (encoded in JSON) to stdout.
 The CLI must return an exit code of 0 on success and non-zero in case of failure.
 
+== Input format
+
+[source,json]
+....
+{
+  "contents": "...",
+  "path": "/a/path/to/the/input/file",
+  "type": "block" # "block" or "inline"
+}
+....
+
 == Install
 
 Download the appropriate binary from the release page.