Update README: still maintained after Rails 4.2.
Update README: still maintained after Rails 4.2.

According to @rafaelfranca, this repo almost doesn't have issues and
it's hard to break, so we can keep on maintaining it even now that
Rails 4.2 is out.

Once we want to remove support, we will probably announce it with an
official statement in a blog post.

[ci skip]
nilbus/actionpack-page_caching,rails/actionpack-page_caching