Release Checklist (#1834)

To help make the process easier and more fool-proof, aid delegation and document institutional knowledge.

I'm not entirely sure where this belongs.

docs/release_checklist.md

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+# Release Checklist
+
+1. Check regressions tag.
+2. Check appropriate milestone.
+3. Check GitHub Projects page for staleness.
+4. Update change log.
+5. Create migration guide.
+6. Write blog post.
+7. Update book.
+8. Create tag on GitHub.
+9. Bump `latest` tag to most recent release.
+10. Release on crates.io
+11. Announce on:
+    1. HackerNews
+    2. Twitter
+    3. Reddit: /r/bevy, /r/rust, /r/rust_gamedev
+    4. Discord: Bevy, Game Development in Rust, Rust Programming Language Community
+    5. This Month in Rust Game Development newsletter
+    6. This Week in Rust newsletter