R: Add note concerning rJava support

Closes Homebrew/homebrew#8255.

Signed-off-by: Charlie Sharpsteen <source@sharpsteen.net>

Formula/r.rb

@@ -70,6 +70,9 @@ class R < Formula
     To use this Framework with IDEs such as RStudio, it must be linked
     to the standard OS X location:
       ln -s "#{prefix}/R.framework" /Library/Frameworks
+
+    To enable rJava support, run the following command:
+      R CMD javareconf JAVA_CPPFLAGS=-I/System/Library/Frameworks/JavaVM.framework/Headers
     EOS
   end
 end