Add localization note to browser.properties [r=me]

mobile/locales/en-US/chrome/browser.properties

@@ -132,6 +132,9 @@ tabs.closeButton=Close tabs
 tabs.closeWarningPromptMe=Warn me when I attempt to close multiple tabs
 
 # Homepage
+# LOCALIZATION NOTE: homepage.custom is the text displayed on the selector button if
+# the user selects a webpage to be the startpage. We can't display the entire URL
+# or webpage title on the menulist, so we just display "Custom"
 homepage.custom=Custom
 
 # Page Actions