Add instructions to use context.WithCancel() to documentation

[Baggerone]

Maybe we just had a strange way of using your library (with gobuffalo).

When we tried to call cache.Get(), we ended up getting a panic (invalid memory address or nil pointer dereference)
down in pool.(*ConnPool).waitTurn line 273

Apparently the context we were using and passing in to cache.Get() didn't have a proper Done() method.

Changing our code to have something like this and sending in that context seemed to fix it.

	ctb := context.Background()
	ctx, _ := context.WithCancel(ctb)	

[vmihailenco]

You can remove ctx, _ := context.WithCancel(ctb). All you need to do is pass a non-nil context which is done with ctb := context.Background(). Nil context.Context is disallowed in Go.