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