README: Add shields badge with link to documentation #169

Job	Run time
check / clang-checks	1m 38s
check / editor-config	7s
build-os-matrix	3s
build (oraclelinux:9) / build	1m 59s
rpm-build (oraclelinux:9) / rpm-build	5m 40s
make-tests (oraclelinux:9) / tests	1m 48s
rpm-install (oraclelinux:9) / rpm-install	1m 19s
	12m 34s

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