Added pool.end()

As brought up in https://github.com/felixge/node-mysql/issues/911

Please let me know if you would like me to adjust...
Thank you. Do you have a proposed / usual way of dealing with this?

--
Dan Bernardic
My contact information is at http://gravatar.com/dbernar1

> On Sep 29, 2014, at 5:43 AM, Christian Tellnes <notifications@github.com> wrote:
> 
> In Readme.md:
> 
> > @@ -320,6 +320,22 @@ being taken from the top of the pool and returning to the bottom.
> >  When a previous connection is retrieved from the pool, a ping packet is sent
> >  to the server to check if the connection is still good.
> >  
> > +## Terminating all the connections in a pool
> > +
> > +When you are done using the pool, you have to end all the connections or your
> > +script will hang until the db server closes them. To end all the connections
> > +in the pool, use
> > +[pool.end()](https://github.com/felixge/node-mysql/blob/master/lib/Pool.js#L121)
> This link is damned to go wrong fast when thing changes. It probably is already.
> 
> —
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