Add a `RELEASING.md` file #1243

Job	Run time
cargo-deny	1m 20s
format	1m 20s
clippy-check	4m 52s
msrv	5m 24s
coverage-main	2m 2s
coverage-tests	2m 9s
check-minimal	1m 20s
coverage-tests (server_system)	1m 42s
Documentation on Github Pages	1m 28s
coverage-tests (client_system)	2m 43s
coverage-tests (client_system, server_system)	2m 7s
publish	5m 58s
	32m 25s

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