readme: document current maintenance status

See #561

README.md

@@ -1,3 +1,11 @@
+Not maintained
+==============
+
+This repository is not under active maintenance because current organization owners are not reachable. See https://github.com/whoosh-community/whoosh/issues/561 for more info.
+
+The development will continue at https://github.com/Sygil-Dev/whoosh-reloaded.
+
+
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