Added example for passing custom props

[bjornharrtell]

I ended using the method described at #1375 which was never merged into docs.

I think it's useful for cases where you derive some state in a parent (that might be reactive) that nested route components are dependent on.

I hope it's a supported case and that it deserves a more prominent place in the docs.

Thanks to @MilosPaunovic for the original snippet.

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[posva]

Thanks I had to merge locally