Update usage.rst

[wwflower]

I found this very helpful in cleaning up some legacy code so I thought I'd capture the use case.
Document basic use of Proxy and using it for lazy class instantiation.

[ionelmc]

There's exactly that in the https://github.com/ionelmc/python-lazy-object-proxy/blob/master/README.rst

Perhaps that page should be removed completely.

[wwflower]

I think the specific information about using it for object instantiation is helpful. I messed around a bit and found I had to use a function to do the instantiation.

I'm new to github, but I would expect "usage" to tell me how to use it. The files do seem like they could be combined. I suggest you follow whatever the convention is.