add deprecation message

Author: asottile

README.md

@@ -1,3 +1,13 @@
+# DEPRECATED
+
+it turns out multiple go shared objects in a single process is not supported
+
+it likely broke in [go 1.21] and there is no intention to fix it :(
+
+[go 1.21]: https://github.com/golang/go/issues/65050#issue-2074509727
+
+___
+
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