readme: add completion info

README.md

@@ -19,6 +19,9 @@ _2)_ It eases handling of loading and saving sessions by only working with a
       :SLoad    load a session
       :SSave    save a session
 
+NOTE: Both commands can also take session names directly as an argument. You can
+also make use of completion via `<c-u>` and `<tab>`.
+
 The default settings are pretty sane, so it should work without any
 configuration.