Contributing to Geronimo's documentation is as easy as easy can get!
We use a Confluence wiki to develop the project's documentation, then we massage the wiki content with a Confluence plugin to generate a custom HTML version. That is what you see when you access http://cwiki.apache.org/geronimo
So, if you are getting this far you are probably monitoring the User and the Development mailing lists already. The issues discussed on these list are mostly driving what topics need to be covered by the documentation. So, the mailing lists are a great starting point for discussing what areas you would like to see covered or can directly contribute to. User experiences are a great asset for the documentation.
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So, c'on in, jump into the mailing lists and bring your ideas forward!