Use - argument instead of missing argument for stdin

Will Alexander · Will Alexander · commit 4cd375d808c0 · 2015-10-20T21:29:37.000+01:00
diff --git a/README.md b/README.md
@@ -23,3 +23,5 @@ Usage
 -----
 
     hostsblock [LIST]
+
+If the list argument is [code]-[/code], hostsblock will read from stdin.
