move upgrade notes to separate file

UPGRADE_NOTES.md

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+# Upgrade Notes
+
+## 2024.9.0
+
+### Following Feed
+
+When upgrading an existing instance to version 2024.9.0, the Following Feed will initially be empty.
+The feed will gradually fill as new posts federate, but it may be desirable to back-fill the feed with existing data.
+This database script will populate the feed with the latest post of each type for all users, ensuring that data is fully populated after the update.
+Run this after migrations but before starting the instance.
+Warning: the script may take a long time to execute!
+
+```postgresql
+INSERT INTO latest_note (user_id, note_id, is_public, is_reply, is_quote)
+SELECT
+	"userId" as user_id,
+	id as note_id,
+	visibility = 'public' AS is_public,
+	"replyId" IS NOT NULL AS is_reply,
+	(
+		"renoteId" IS NOT NULL
+			AND (
+			text IS NOT NULL
+				OR cw IS NOT NULL
+				OR "replyId" IS NOT NULL
+				OR "hasPoll"
+				OR "fileIds" != '{}'
+			)
+		) AS is_quote
+FROM note
+WHERE ( -- Exclude pure renotes (boosts)
+				"renoteId" IS NULL
+					OR text IS NOT NULL
+					OR cw IS NOT NULL
+					OR "replyId" IS NOT NULL
+					OR "hasPoll"
+					OR "fileIds" != '{}'
+				)
+ORDER BY id DESC -- This part is very important: it ensures that we only load the *latest* notes of each type. Do not remove it!
+ON CONFLICT DO NOTHING; -- Any conflicts are guaranteed to be older notes that we can ignore.
+```