Add "Release flow" to README.rdoc #383

Job	Run time
ruby-versions / Generate Ruby versions	3s
test (2.5, ubuntu-latest)	41s
test (2.5, macos-latest)	1m 3s
test (2.5, windows-latest)	2m 45s
test (2.6, ubuntu-latest)	55s
test (2.6, macos-latest)	1m 6s
test (2.6, windows-latest)	3m 35s
test (2.7, ubuntu-latest)	34s
test (2.7, macos-latest)	1m 10s
test (2.7, windows-latest)	2m 39s
test (3.0, ubuntu-latest)	19s
test (3.0, macos-latest)	36s
test (3.0, windows-latest)	2m 0s
test (3.1, ubuntu-latest)	22s
test (3.1, macos-latest)	33s
test (3.1, windows-latest)	1m 54s
test (3.2, ubuntu-latest)	30s
test (3.2, macos-latest)	36s
test (3.2, windows-latest)	3m 36s
test (head, ubuntu-latest)	39s
test (head, macos-latest)	56s
test (head, windows-latest)	2m 5s
test (jruby, ubuntu-latest)	6m 40s
test (jruby-head, ubuntu-latest)	6m 22s
test (truffleruby, ubuntu-latest)	4m 58s
test (truffleruby-head, ubuntu-latest)	4m 4s
	50m 41s

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