let's link to the gem README not to the upgrading guide. [ci skip]

The PR #13767 added link to the upgrading guide to explain
details about activerecord_deprecated-finders. However the link target
features a stack of changes not releated at all. Also the relevant details
are not very informative.

I think we better link to the README so people can see what the gem is about
and how to use it.

/cc @chancancode @zzak

guides/source/4_1_release_notes.md

@@ -411,8 +411,7 @@ for detailed changes.
 * Remove implicit join references that were deprecated in 4.0.
 
 * Removed `activerecord-deprecated_finders` as a dependency.
-  Please see [upgrading
-  guide](upgrading_ruby_on_rails.html#upgrading-from-rails-3-2-to-rails-4-0-active-record)
+  Please see [the gem README](https://github.com/rails/activerecord-deprecated_finders#active-record-deprecated-finders)
   for more info.
 
 * Removed usage of `implicit_readonly`. Please use `readonly` method