Update README.md

I think users have to add in 'project/plugins.sbt' :

addSbtPlugin("com.jamesward" % "play-auto-refresh" % "0.0.11")

and NOT :

addSbtPlugin("com.jamesward" %% "play-auto-refresh" % "0.0.11")
Merge pull request #28 from gpoulet/patch-1

Update README.md