Update changelog for v3.1.1 release
## HEAD (Unreleased)
+## 3.1.1 (May 5, 2021)
+
- Avoid circular dependencies in NodeJS SDK modules (https://github.com/pulumi/pulumi-kubernetes/pull/1558)
- Update pulumi dependencies v3.2.0 (https://github.com/pulumi/pulumi-kubernetes/pull/1564)
