Update README.md

Author: Gankra

README.md

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+**WARNING: THIS PROJECT IS IN MAINTENANCE MODE, DUE TO INSUFFICIENT MAINTAINER RESOURCES**
+
+It works fine, but will generally no longer be improved.
+
+We are currently only accepting changes which:
+
+* keep this compiling with the latest versions of Rust or its dependencies.
+* have minimal review requirements, such as documentation changes (so not totally new APIs).
+
+------
+
+
 A HashMap wrapper that holds key-value pairs in insertion order.
 
 Documentation is available at https://contain-rs.github.io/linked-hash-map/linked_hash_map.