update Documentation section in README
update Documentation section in README

1. rake doc does not work; use rake rdoc.
2. gem mislav-hanna does not exist; use hanna.
3. rake rdoc 'depends' on hanna; no need of RDOCOPTS

increments/libxml-ruby,increments/libxml-ruby,sferik/libxml-ruby,sferik/libxml-ruby,sferik/libxml-ruby,sferik/libxml-ruby,sferik/libxml-ruby,xml4r/libxml-ruby,increments/libxml-ruby,xml4r/libxml-ruby,xml4r/libxml-ruby,sferik/libxml-ruby