Update README to remove development branch warning

This is a stable branch, and we are already seeing it!

Author: okuramasafumi

README.md

@@ -1,6 +1,3 @@
-:warning:  You are viewing the development's branch version of README which might contain documentation for unreleased features.
-For the README consistent with the latest released version see https://github.com/rack/rack-attack/blob/6-stable/README.md.
-
 # Rack::Attack
 
 *Rack middleware for blocking & throttling abusive requests*