add automatic hash function creation

Since this is my first contribution to boost, please check if the copyright notices and documentation are up to par.  Thanks!
This triggers an error: `Cannot find class named 'boost::hash'`

I would suggest following what _"Adapted"_ and other sections do. That is, display `boost::hash` as code instead of a `classref`, and add a _"See Also"_ section pointing to the _Boost.Hash library_.