added comment in readme about using jruby

README.md

@@ -61,7 +61,10 @@ to_hash() is more flexible and produces more consistent output so it has a
 preference over the to_json() method. If neither the to_json() or to_hash()
 methods exist then the Oj internal Object variable encoding is used.
 
-Oj is compatible with Ruby 1.8.7, 1.9.2, 1.9.3, JRuby, RBX, and the latest 2.0dev.
+Oj is compatible with Ruby 1.8.7, 1.9.2, 1.9.3, JRuby, RBX, and the latest
+2.0dev. Note that JRuby now disables support for extentions by default. JRuby
+can be build with extensions enabled. Check the documenation for JRuby
+installs in your environment.
 
 Oj is also compatible with Rails. Just make sure the Oj gem is installed and
 [multi_json](https://github.com/intridea/multi_json) will pick it up and use it.