add p4merge caveats as recommended in #217

Casks/p4merge.rb

@@ -5,4 +5,10 @@ class P4merge < Cask
   sha1 'b082af5840282bcff5c2445c586316f72cf71551'
 
   link :app, 'p4merge.app'
+
+  def caveats; <<-EOS.undent
+    You can set up git to use p4merge as a merge tool by following the instructions available here:
+      https://gist.github.com/henrik242/1510148
+    EOS
+  end
 end