update cli on defining custom formats

src/main.rs

@@ -59,7 +59,7 @@ fn main() {
         .help(
             "The output format. Valid built-in formats are ical, csv, json, \
             ids, and text. Documentation on defining custom formats can be \
-            found in the README included in this distribution."
+            found at https://gitlab.com/categulario/tiempo"
         );
 
     let sheet_arg = Arg::with_name("sheet")