Add release note

Author: edolstra

doc/manual/rl-next/self-submodules-attr.md

@@ -0,0 +1,12 @@
+---
+synopsis: "`inputs.self.submodules` flake attribute"
+prs: [12421]
+---
+
+Flakes in Git repositories can now declare that they need Git submodules to be enabled:
+```
+{
+  inputs.self.submodules = true;
+}
+```
+Thus, it's no longer needed for the caller of the flake to pass `submodules = true`.