content/advanced-concepts: fill me in

[odeke-em]

This is an issue to add a reminder that we need help filling out the "Advanced Concepts" section https://opencensus.io/advanced-concepts/

Please feel free to pitch in everyone but for starters I'll page @rakyll @basvanbeek @bogdandrutu

[basvanbeek]

I'd like to pick up context propagation but since I'll be on vacation the next two weeks someone can take it over if they have time for it now.