Document how applications can control the initial render with Suspense

[Kingdutch]

As outlined in #67 the suspense boundary of the router has been removed. This provides the application developers the power (but also the responsibility) on what should be rendered before we start sending data to the client. This is mentioned in the React Docs briefly but desires further explanation.

[zth]

Would you like to take a stab at this @Kingdutch ?

[Kingdutch]

Yeah this is on my list of things I'd like to tackle, though not a priority at the moment.