Add CalVer deprecation notice to README (#373)

Added deprecation notice for CalVer versioning scheme.

Author: mimartin12

README.md

@@ -1,4 +1,13 @@
 # Bitwarden Helm Charts Repository
+> [!IMPORTANT]
+> **CalVer Deprecation Notice**
+> 
+> The CalVer versioning scheme for the Bitwarden self-host chart will be deprecated on **November 13, 2025**.
+> 
+> After this date, only SemVer versions will be supported and released.
+> 
+> For more information, see the chart's [NOTES.txt](https://github.com/bitwarden/helm-charts/blob/main/charts/self-host/templates/NOTES.txt).
+
 
 - [Self-Host](charts/self-host/README.md)
 - [Secrets Manager Operator](charts/sm-operator/README.md)