Explain "Sampled" header as discussed in #20
Explain "Sampled" header as discussed in #20

openzipkin/openzipkin.github.io,basvanbeek/openzipkin.github.io,openzipkin/openzipkin.github.io,openzipkin/openzipkin.github.io,basvanbeek/openzipkin.github.io,basvanbeek/openzipkin.github.io