Documents are kept on Google Docs but we would like to attach them to a list in Trello for reference or to work on the document. How does this work?
Check out the work being done by Zapier.com. It is not free, but it is a very powerful service for integrating many different web services.
Like Bobby Grace said there is no real Google Docs <-> Trello.com integration, at least not yet. In time there could be as Trello.com is working on their API and maybe someone will implement some kind of integration.
Meanwhile you can either link to the document or to a specific place in the document. If you want to link to a specific place, you can insert a bookmark in your document and link to that in Trello. To insert a bookmark:
Insert and choose
Bookmark. To get the link, click the blue banner and right click the
Link and choose
Save link address.
You can include a hyperlink to the document by using the format:
[ display text](url) [this is a link]( http://google.com )
Which will look like this:
And if you create a bookmark as @Benregn suggested, you can link to a particular part of the gDoc.