Improved reference to internals list for code contributions.

working on the MySQL code. If you plan to contribute to MySQL

development, want to have access to the bleeding-edge in-between

versions code, or just want to keep track of development, follow the

instructions in @xref{Installing source tree}.  If you are interested in MySQL

internals, you should also subscribe to @email{internals@@lists.mysql.com}.

This is a relatively low traffic list, in comparison with 

@email{mysql@@lists.mysql.com}.

instructions in @xref{Installing source tree}.  If you are interested

in MySQL internals, you should also subscribe to our @code{internals}

mailing list. This list is relatively low traffic. For details on how

to subscribe, please see @ref{Mailing-list}.

@menu

* MySQL threads::               MySQL threads