document how to use opal browser

[catmando]

Its unclear what you need to do to do simple stuff like

Window.location.path = 'some new path'

(the answer is you have to first import browser/window)

but for these basic things that rely on underlying opal libs, we should have some examples for the basic cases.

[Tashows]

I was trying to figure out how to call window for so loooong. Thanks!