shell lib: Add example how to run C child #268

Job	Run time
job (debian:stable, gcc, i386)	6m 44s
job (debian:stable, ppc64el, powerpc64le-linux-gnu-gcc, 1, out, cross-compile)	5m 8s
job (debian:stable, arm64, aarch64-linux-gnu-gcc, 1, out, cross-compile)	5m 17s
job (debian:stable, s390x, s390x-linux-gnu-gcc, 1, out, cross-compile)	4m 52s
job (alpine:latest, gcc, asciidoctor)	4m 42s
job (debian:stable, gcc, out, minimal)	3m 23s
job (opensuse/archive:42.2, gcc)	5m 53s
job (quay.io/centos/centos:stream9, gcc, asciidoc-pdf, out)	4m 19s
job (debian:testing, gcc, asciidoctor-pdf)	6m 55s
job (debian:oldstable, clang)	7m 32s
job (fedora:latest, clang, 1, asciidoctor-pdf)	6m 22s
job (opensuse/leap:latest, gcc, asciidoc-pdf)	6m 24s
job (debian:oldstable, gcc, asciidoctor)	6m 7s
job (debian:testing, clang, asciidoctor-pdf)	8m 9s
job (ubuntu:jammy, gcc, asciidoctor, out)	4m 19s
job (ubuntu:bionic, gcc, asciidoc-pdf)	5m 41s
	1h 31m 47s

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