Define configuration and CLI contracts (#2)

README.md

@@ -24,6 +24,9 @@ and `gpg`—are forbidden; compatibility is implemented in Rust.
 Candidate libraries and the pending project-license decision are tracked in
 [`DEPENDENCIES.md`](DEPENDENCIES.md).
 
+The shared TOML schema, path rules, editor precedence, and HTTPS remote format
+are documented in [`docs/configuration.md`](docs/configuration.md).
+
 ## Project layout
 
 ```text