Documentation is rare! But very vital for extending current (0.9) nutch. Current docs on the wiki for 0.7 plugin development was good but it doesn't apply to 0.9 and new developers who are joining directly 0.9 find the 0.7 documentation not enough. A more practical plugin writing documentation for 0.9 is desired also exposing the plugin principals in practical terms i.e. extension points and libs etc. furthermore it would be good to provide some best practice example i.e.
look for the lib you are planning to use if its already in lib folder and maybe that version of the external lib is good for the plugin dev rather then using another version things like that..