Update README.md for 6.0.0 (#50)

Automated update of `README.md` for release 6.0.0

Co-authored-by: keith <keith@users.noreply.github.com>

README.md

diff --git a/README.md b/README.md
index 27d60e8..f3e4865 100644
--- a/README.md
+++ b/README.md
@@ -48,10 +48,10 @@
 
 http_archive(
     name = "buildifier_prebuilt",
-    sha256 = "ecef8f8c39eaf4f1c1604c677d232ade33818f898e35e7826e7564a648751350",
-    strip_prefix = "buildifier-prebuilt-5.1.0.2",
+    sha256 = "95387c9dded7f8e3bdd4c598bc2ca4fbb6366cb214fa52e7d7b689eb2f421e01",
+    strip_prefix = "buildifier-prebuilt-6.0.0",
     urls = [
-        "http://github.com/keith/buildifier-prebuilt/archive/5.1.0.2.tar.gz",
+        "http://github.com/keith/buildifier-prebuilt/archive/6.0.0.tar.gz",
     ],
 )