Add strict custom field registry

specs/versions/0.1/commands.md

@@ -79,6 +79,17 @@ Build a stable GSP index.
 gsp index [--root .] [--out index.json]
 ```
 
+## fields
+
+List or validate the project custom field registry.
+
+```bash
+gsp fields list [--root .] [--out fields.json]
+gsp fields validate [--root .] [--out fields-report.json]
+```
+
+`gsp validate` also validates custom fields by default.
+
 ## trace
 
 Inspect relation chains from one GSP id.