Update template for releases to include upgrade instructions

[fulldecent]

When releasing a new version of Yarn Berry here, on the announce page, please link to an explanation of how to update to the latest yarn in your own projects.

This information will be relevant to people reading the release page.

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In contrast, if you Google "how to update yarn in my project" the experience is actively unhelpful.

If you ask ChatGPT "how to update yarn in my project" the experience is not good.

If you ask "how to update yarn in my project" in TikTok the experience is irrelevant.