Add instructions for opset change contributions to CONTRIBUTING.md (#…

…1921)

Although the RFC process under OpenXLA is not fully standardized yet,
there have been a few changes to StableHLO lately which had success
using a similar process. This PR adds some documentation on the steps
they followed to `contributing.md`.

CONTRIBUTING.md

@@ -21,3 +21,62 @@ All submissions, including submissions by project members, require review. We
 use GitHub pull requests for this purpose. Consult
 [GitHub Help](https://help.github.com/articles/about-pull-requests/) for more
 information on using pull requests.
+
+## StableHLO Opset Changes
+
+All changes to the StableHLO opset including new ops, types, or attributes must
+be reviewed via an RFC. We aim for StableHLO opset changes to take ~2 weeks
+if feedback is actively addressed. This allows adequate time for the community
+to review all proposals.
+
+### 1. Write an RFC
+
+An RFC should outline the proposed spec changes, as well as the rationale, and
+alternatives considered if relevant. This can be shared as a markdown file in
+the [`rfcs/`](https://github.com/openxla/stablehlo/tree/main/rfcs) directory and
+shared as a PR.
+
+For example, see the [`collective_broadcast` RFC](https://github.com/openxla/stablehlo/pull/1809).
+
+### 2. Notify OpenXLA Discuss
+
+To signal boost your RFC, post on [OpenXLA Discuss](https://groups.google.com/a/openxla.org/g/openxla-discuss).
+This will ensure the proper reviewers see the RFC. While there is no formal
+process for posts, we tend to recommend keeping RFC discussion on the PR to keep
+feedback centralized in the repository.
+
+For example, see the [`collective_broadcast` post](https://groups.google.com/a/openxla.org/g/openxla-discuss/c/Q7JFyoiVFPU/m/dUH_LmJlCgAJ).
+
+### 3. Work with project maintainer for final approval
+
+As denoted in [`governance.md`](https://github.com/openxla/stablehlo/blob/main/docs/governance.md),
+while we work towards instating StableHLO module maintainers, the interim review
+process requires approval from Google project maintainers. A member of the
+StableHLO team will help drive final approval.
+
+### 4. Send PRs for the opset change
+
+Once an RFC is approved, PRs which implement the approved proposal may be sent,
+reviewed, and merged.
+
+A few things to consider when adding new features:
+
+- Spec, Type Inference, Verifiers: Steps for adding to [`spec.md`](https://github.com/openxla/stablehlo/blob/main/docs/spec.md)
+as well as related op implementation can be found in
+[`spec_checklist.md`](https://github.com/openxla/stablehlo/blob/main/docs/spec_checklist.md).
+- Verifiers: Steps for modifying or implementing op verifiers can also be found
+in the spec checklist.
+- Type Inference: Type inference design principles and testing details can be
+found in [`type_inference.md`](https://github.com/openxla/stablehlo/blob/main/docs/type_inference.md).
+- Compatibility: Tips on managing forward/backward compatibility are in
+[`vhlo.md`](https://github.com/openxla/stablehlo/blob/main/docs/vhlo.md#contributing-incompatible-changes).
+- Reference: Steps for adding interpreter support can be found in
+[`reference_checklist.md`](https://github.com/openxla/stablehlo/blob/main/docs/reference_checklist.md).
+- Tests: For each of the above modifications, consider positive and negative
+test cases.
+
+Some examples to help guide changes:
+
+- Adding a new op: [`collective_broadcast`](https://github.com/openxla/stablehlo/pull/1856).
+- Adding new types: [`f8E4M3FNUZ` and `f8E5M2FNUZ`](https://github.com/openxla/stablehlo/pull/1379).
+- Expanding type support: [Quantized `ReduceOp`](https://github.com/openxla/stablehlo/pull/1796).

docs/spec_checklist.md

@@ -5,7 +5,8 @@ specification. At the moment, these changes typically involve checking multiple
 things in multiple sources, so this document summarizes them all to simplify
 reviews:
 
-  1. Check that the "Specification" column in status.md says "yes".
+  1. Check that the "Specification" column in status.md says "yes", add a row if
+     adding a new op.
   1. Check if the section title matches the op's mnemonic in
      [the ODS](https://github.com/openxla/stablehlo/blob/main/stablehlo/dialect/StablehloOps.td).
   1. Check if the "Semantics" section matches XLA's