add example to slsqp itself

With discussion on scipy mailing list on constrained optimization, I was wondering what the trust based nonlin leastsq would bring extra to scipy as slsqp seems present. 

To make it more visible what slsqp can do, I would suggest adding an example also in the reference, and not only in the tutorial. Example taken here is based on the __main__ section in slsqp.py and different from the tutorial which uses minimize directly (which I personally don't like as I rather work with the core functions myself so like to see an example of that).