docs: add best practices guide for API extensions

sig-network/api-extensions-best-practices.md

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+# API Extensions - Best Practices
+
+It is sometimes desireable to build new networking features as [CRDs] rather
+than [in-tree]. Reasons may include (but aren't limited to):
+
+* A need to move at a different pace than the core project
+* Experimentation and highly iterative development
+* The feature is useful to multiple parties, but is niche
+* the API doesn't or can't have a default implementation in core
+
+Whatever the case may be, we have multiple SIG Network sub-projects that have
+taken this approach for various reasons:
+
+* [Gateway API]
+* [Network Policy]
+* [Multi Network]
+
+This document intends to share guidelines and best practices for how to create
+and manage these kinds of "official, but add-on and optional" features, which
+throughout this document we'll refer to as **Official CRDs**.
+
+> **Note**: **This is not a standard**, but **SIG Network projects should try
+> and follow this guidance** unless they have some good reasons not to, as the
+> guidance here comes from the experience of those who have walked this road
+> before. This guidance might be applicable to some other SIGs, but as of the
+> time of writing none except SIG Network have reviewed this.
+
+> **Note**: This is **not a comprehensive guide**.
+
+[CRDs]:https://kubernetes.io/docs/concepts/extend-kubernetes/api-extension/custom-resources/
+[in-tree]:https://github.com/kubernetes/api
+[KEP]:https://github.com/kubernetes/enhancements/blob/master/keps/README.md
+[Gateway API]:https://github.com/kubernetes-sigs/gateway-api
+[Network Policy]:https://github.com/kubernetes-sigs/network-policy-api
+[Multi Network]:https://github.com/kubernetes-sigs/multi-network
+
+## Objectives
+
+At a high level, SIG Network sub-projects that deploy as [CRDs] will need to
+deal with (at least) the following dimensions:
+
+* Enhancement Proposals
+* Developing CRDs
+* Delivering CRDs
+* CRD Lifecycle Management
+* Conformance Tests & Reports
+* Documentation
+* Community & Evangelism
+
+In the following section we'll talk about each of these in-depth, including
+some lessons learned by existing or historical sub-projects.
+
+[CRDs]:https://kubernetes.io/docs/concepts/extend-kubernetes/api-extension/custom-resources/
+
+### Enhancement Proposals
+
+Projects which produce optional [Custom Resource Defintion (CRD)][CRD]-based
+APIs should provide a project-specific enhancement proposal system. You can see
+examples of using project-specific enhancement proposals with some of the
+current sub-projects:
+
+* [Gateway Enhancement Proposals (GEPs)][geps]
+* [Network Policy Enhancement Proposals (NPEPs)][npeps]
+
+Key characteristics of this pattern are:
+
+* an easy to find directory at the root of repositories, such as `geps/`, with
+  references provided from the `README.md`.
+* a readme for the enhancement process (we recommend using GEP as a template,
+  and we recommend a motivation focused proposal process (see more below)).
+* a project board which organizes these GEPs and helps provide visibility into
+  the project's priorities and roadmap
+* trackable issues and PRs for every proposal that go through phases (e.g.
+  (`provisional`, `implementable`, `experimental`, e.t.c.)
+
+We strongly recommend this pattern (unless there's some very compelling reason
+reason not to use it for any particular project).
+
+> **Note**: When starting new projects, we advise having enhancement proposals
+> preceed any code whatsoever, for posterity and to ensure time for all
+> stakeholders in the community are represented.
+
+> **Note**: Taking this approach doesn't mean complete autonomy. Any GA APIs
+> (version is v1 or greater) with a `*.k8s.io` group **MUST** go through the API
+> review process with the SIG Network leads for any content considered
+> stable/standard/GA. The use of any non-`*.k8s.io` group for APIs published by
+> official SIG Network sub-projects or working groups needs to be ratified with
+> SIG Network leads first. Experimental features, alpha, beta, e.t.c. can be
+> developed without that additional oversight until they are promoted to GA.
+
+[KEP]:https://github.com/kubernetes/enhancements/blob/master/keps/README.md
+[geps]:https://github.com/kubernetes-sigs/gateway-api/blob/main/geps/overview.md
+[npeps]:https://github.com/kubernetes-sigs/network-policy-api/tree/main/npeps
+
+#### Motivation Focused Enhancement Proposals
+
+It can be very easy to bring a new proposal to the table, and start with a huge
+GEP and maybe a ton of upfront code. In our experience however having too much
+up front, and in particular coming right to the table with an implementation
+can cause a proposal to get gridlocked very quickly.
+
+**We strongly recommend a "What, why and who before how" approach**. The first
+PR which adds an enhancement proposal should establish:
+
+* **What?** - What do we want to do, at a high level? A summary and/or specific
+  high-level goals to achieve.
+* **Why?** - The motivation, the user stories, the reason we want to do this.
+* **Who?** - Every proposal stands a better chance of succeeding if there are
+  multiple aligned parties interested in it, and collaborating on it upfront.
+
+Without any indication of _how_ we accomplish it. This, plus a process of steps
+for graduation (e.g. `provisional` -> `implementable` -> `experimental` ->
+`standard`) allows the community to work more iteratively, and work on alignment
+with the principles before spending any time in the implementation details.
+
+### Developing CRDs
+
+TODO
+
+### Delivering CRDs
+
+TODO
+
+### CRD Lifecycle Management
+
+TODO - installing, and managing CRDs. How this should work on platforms, who's
+       responsible (and trying to reduce responsibility on the cluster operator)
+       Also need to talk about interop between multiple dependent projects.
+
+### Conformance Tests & Reports
+
+TODO - framework for testing and reporting
+
+### Documentation
+
+TODO
+
+## Important Notes
+
+> **Note**: At the time of writing, we have no extension APIs that have
+> transferred themselves in-tree after doing development iterations
+> out-of-tree. This is uncharted territory at present, though we are not
+> opposed to it happening if it seems appropriate.