Postgresql - add link about build failures

Formula/postgresql.rb

@@ -85,6 +85,10 @@ class Postgresql <Formula
 
   def caveats
     s = <<-EOS
+If builds of Postgresl 9 are failing and you have version 8.x installed,
+you may need to remove the previous version first. See:
+  http://github.com/mxcl/homebrew/issues/issue/2510
+
 To build plpython against a specific Python, set PYTHON prior to brewing:
   PYTHON=/usr/local/bin/python  brew install postgresql
 See: