add generics policy draft

as discussed in the last SIG Architecture meeting

Author: BenTheElder

sig-architecture/generics.md

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+# Background
+
+In general, Kubernetes has not restricted using new Go features, we've quickly
+adopted new standard library types and methods and will continue to do so.
+
+Generally the latest stable go release is in use on the main development branch.
+This includes all of the staging libraries (client-go etc.) that originate in the
+main [kubernetes/kubernetes](https://github.com/kubernetes/kubernetes) repository.
+
+In Kubernetes v1.24 we shipped Go 1.18 (which adds support for generics)
+rather late in the release cycle, so we temporarily prohibited using generics
+in case we ran into other issues and needed to roll back to unblock the release.
+Now that v1.24.0 is out, use of generics should be allowed.
+
+# Generics Policy
+
+Generics may be used in Kubernetes starting in v1.25, with the following restrictions 
+applying only until v1.24 is [out of support][version-support]:
+
+- Generics should **not** be used in Kubernetes libraries used across multiple Kubernetes
+versions, that is the non "staged" libraries like:
+  - [k8s.io/utils](https://github.com/kubernetes/utils)
+  - [sigs.k8s.io/yaml](https://github.com/kubernetes-sigs/yaml)
+  - [k8s.io/klog](https://github.com/kubernetes/klog)
+  - etc.
+
+- Generics should be **avoided** when writing Kubernetes bug fixes that are likely to be backported, to streamline cherry-picking to older release branches.
+
+These restrictions should be considered lifted when v1.24 is out of support.
+
+## Recommendations for Reviewers
+
+- Consider if proposed generics pull requests improve maintainability and readability.
+
+- The current generics implementation is known to have some performance issues
+depending on usage: consider requesting benchmarks before / after the changes.
+
+[version-support]: https://kubernetes.io/releases/patch-releases/#support-period