Add header to bincode advisory

Author: EthanPlant

crates/bincode/RUSTSEC-0000-0000.md

@@ -9,6 +9,7 @@ informational = "unmaintained"
 [versions]
 patched = []
 ```
+# Bincode is unmaintained
 
 Development on bincode has ceased permanently, and updates will only be made in the event of a CVE. Projects are encouraged to migrate to alternatives such as `postcard` if possible. If compatibilty with the bincode format is required, consider using `wincode`.