Add library unmaintained notice

Author: includesec-ltennant

README.md

@@ -1,4 +1,7 @@
 # SafeURL for Scala
+
+**Note: The SafeURL libraries are no longer maintained and we recommend considering other SSRF mitigation approaches alongside application-layer SSRF protection libraries. See our [2023 blog post](https://blog.includesecurity.com/2023/03/mitigating-ssrf-in-2023/) for more details.**
+
 ### Originally Ported by [@saelo](https://github.com/saelo)
 
 ## Overview