Installation docs are out of date

[mslinn]

This is the magic incantation for Scala 2.11.6:

addCompilerPlugin("org.scalamacros" % "paradise" % "2.1.0-M5" cross CrossVersion.full)

The docs will be even more out of date when Macro Paradise becomes available for Scala 2.11.7
scalamacros/paradise#71

[mslinn]

Macro Paradise is now available for Scala 2.11.7 so my previous incantation works. This updated info should be in the README.