Document the new release process for Rustup #1062

Job	Run time
build-freebsd-master	0s
build-all-features (x86_64-unknown-linux-gnu)	7m 4s
build-linux-master	0s
build-freebsd-stable	0s
build-linux-pr (dev, x86_64-unknown-linux-gnu)	10m 22s
build-linux-stable	0s
build-macos-aarch64 (dev, aarch64-apple-darwin)	15m 15s
build-linux-pr (dev, armv7-unknown-linux-gnueabihf)	3m 6s
build-macos-x86_64 (dev, x86_64-apple-darwin)	17m 26s
build-macos-aarch64 (release, aarch64-apple-darwin)	3m 34s
build-linux-pr (dev, aarch64-linux-android)	3m 0s
build-macos-x86_64 (release, x86_64-apple-darwin)	6m 0s
build-windows-master	0s
build-linux-pr (release, x86_64-unknown-linux-gnu)	5m 10s
build-windows-pr (dev, x86_64-pc-windows-msvc)	19m 59s
build-linux-pr (release, armv7-unknown-linux-gnueabihf)	4m 20s
build-windows-stable	0s
build-linux-pr (release, aarch64-linux-android)	3m 48s
build-windows-pr (dev, aarch64-pc-windows-msvc)	13m 18s
check	2m 54s
build-windows-pr (dev, x86_64-pc-windows-gnu)	6m 46s
doc	7s
build-windows-pr (release, x86_64-pc-windows-msvc)	7m 55s
build-windows-pr (release, aarch64-pc-windows-msvc)	7m 45s
build-windows-pr (release, x86_64-pc-windows-gnu)	9m 34s
conclusion	1s
	2h 27m 24s

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