store: Document `add-to-store' hack for `fixed?'.

* guix/store.scm (add-to-store): Document that `fixed?' must be #t.

guix/store.scm

@@ -414,7 +414,8 @@ (define-operation (add-to-store (string basename)
                                 (boolean recursive?)
                                 (string hash-algo)
                                 (file file-name))
-  "Add the contents of FILE-NAME under BASENAME to the store."
+  "Add the contents of FILE-NAME under BASENAME to the store.  Note that
+FIXED? is for backward compatibility with old Nix versions and must be #t."
   store-path)
 
 (define-operation (build-derivations (string-list derivations))