doc: mention v3 and v4, with a link

Signed-off-by: Alexis Lefebvre <alexislefebvre+github@gmail.com>

Author: alexislefebvre

README.md

@@ -87,6 +87,11 @@ Want to help develop Docker Compose? Check out our
 If you find an issue, please report it on the
 [issue tracker](https://github.com/docker/compose/issues/new/choose).
 
+Where are v3 and v4?
+--------------------
+
+These versions have been skipped, see the [5.0 release](https://github.com/docker/compose/releases/tag/v5.0.0) for a detailed explanation.
+
 Legacy
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