rust: document manual configuration for rustfmt

Currently, rustfmt requires to be enabled per repository. Add a comment
near the current project allowed_list as a reminder.

Test: None
Bug: 160223496
Change-Id: Iecf8d5b693620541a00d8ddc905549652025eed9

rust/config/allowed_list.go

@@ -1,6 +1,10 @@
 package config
 
 var (
+	// When adding a new path below, add a rustfmt.toml file at the root of
+	// the repository and enable the rustfmt repo hook. See aosp/1347562
+	// for an example.
+	// TODO(b/160223496): enable rustfmt globally.
 	RustAllowedPaths = []string{
 		"external/minijail",
 		"external/rust",