Add help text for icon URL

app/views/layers/_form.html.haml

@@ -12,7 +12,7 @@
     = f.text_field :name
   %br
   .field
-    = f.label "Icon URL (optional)"
+    = f.label "Icon URL (optional; maximum 100px wide by 75px high)"
     %br
     = f.text_field :icon_url, :size => 60
   %br