4.x: Update READMEs and add a CONTRIBUTING.md #727

Job	Run time
build	5m 1s
shellcheck	3m 8s
copyright	3m 13s
checkstyle	3m 26s
javadoc	4m 1s
cli/linux-amd64	4m 55s
tests/cli-linux	5m 16s
spotbugs	5m 28s
cli/windows-amd64	5m 45s
tests/cli-functional-linux	6m 37s
tests/archetype-linux	4m 46s
vscode-ext	3m 52s
tests/linker-linux	5m 1s
cli/darwin-amd64	5m 52s
tests/others-linux	5m 49s
tests/cli-windows	6m 25s
tests/cli-functional-windows	7m 22s
tests/archetype-windows	5m 34s
tests/linker-windows	5m 22s
tests/others-windows	7m 20s
cli/binaries	4s
tests/results	6s
gate	1s
	1h 44m 24s

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