message: Add changelog warning
diff: @@ -4,6 +4,10 @@ Changelog
  0.22.0 (2022-09-27)
  -------------------
  +Warning note for upgraders. This release has a database migration on the
  +`oauth2_tokens` table. For most users this is the largest table, some
  +downtime may be expected while the server runs its migrations.
  +
  * #425: Using a `client_secret` is now supported with `authorization_code`,
  and it's read from either the request body or HTTP Basic Authorization
  header.
  