Clarify shortcut

Author: tiimgreen

README.md

@@ -75,7 +75,7 @@ Pressing `w` will bring up the branch selector.
 
 Pressing `s` will select the search bar.
 
-Pressing `y` will change your URL to one which, in effect, freezes the page you are looking at. If this code changes, you will still be able to see what you saw at that current time.
+Pressing `y` __when looking at a file__ (e.g. `https://github.com/tiimgreen/github-cheat-sheet/blob/master/README.md`) will change your URL to one which, in effect, freezes the page you are looking at. If this code changes, you will still be able to see what you saw at that current time.
 
 To see all of the shortcuts for given page type `shift+?`