Update changelog for v3.8.1 release
## HEAD (Unreleased)
+## 3.8.1 (October 8, 2021)
+
- Fix error for helm.Release previews with computed values (https://github.com/pulumi/pulumi-kubernetes/pull/1760)
- Don't require values for helm.Release (https://github.com/pulumi/pulumi-kubernetes/pull/1761)
