Add a retirement note in the README
diff --git a/README.md b/README.md
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+++ b/README.md
@@ -1,3 +1,9 @@
+## Retired project
+
+This Apache James sub-project had been retired on the 30/07/2021.
+
+[Read more](https://www.mail-archive.com/server-dev@james.apache.org/msg70607.html)
+
 ##### Introduction #####
 Hupa is a Rich IMAP-based Webmail application written in GWT.