add release plan #5713

Job	Run time
Test	2m 16s
Build extensions	1m 24s
ember-try (ember-lts-3.16)	1m 11s
ember-try (ember-lts-3.20)	1m 20s
ember-try (ember-lts-3.24)	1m 20s
ember-try (ember-lts-3.28)	1m 13s
ember-try (ember-release)	1m 27s
ember-try (ember-beta)	1m 18s
ember-try (ember-canary)	1m 18s
ember-try (ember-default-no-prototype-extensions)	1m 11s
Publish Firefox extension	22s
Publish Chrome extension	14s
Publish npm package	7s
Publish bookmarklet	0s
	14m 41s

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