Change search box placeholder

Hopefully this makes it more obvious what to do immediately post-install.

// FREEBIE

background.html

@@ -13,7 +13,7 @@
       <span class='socket-status'></span>
       <div class='tool-bar clearfix'>
         <!-- <button class='show-new-conversation'></button> -->
-        <input type='text' class='search' placeholder='Search'>
+        <input type='text' class='search' placeholder='Search for people or groups'>
       </div>
     </div>
     <div class='gutter'>