add multiplexer.md #218

Job	Run time
Book tests	3m 47s
Build target wasm32-unknown-unknown	4m 11s
Bitrot check	1m 37s
Build target wasm32-wasi	4m 5s
Intra-doc links	3m 56s
Rustfmt	14s
Test on ubuntu-latest	9m 41s
Test on i686-unknown-linux-gnu	16m 2s
Test on windows-latest	12m 23s
Test on i686-unknown-linux-gnu with beta features	15m 36s
Test on macOS-latest	12m 57s
Test on i686-unknown-linux-gnu with nightly features	18m 16s
Test on ubuntu-latest with beta features	9m 41s
Test on windows-latest with beta features	12m 27s
Test on macOS-latest with beta features	12m 28s
Test on ubuntu-latest with nightly features	10m 57s
Test on windows-latest with nightly features	13m 44s
Test on macOS-latest with nightly features	12m 37s
	2h 54m 39s

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