docs: add workloads contribution guide

Author: HybridOps Export Bot

CONTRIBUTING.md

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+# Contributing to HybridOps Workloads
+
+This repository holds the public workload baseline for HybridOps.
+Contributions are welcome when they improve a reusable workload, public target,
+or the validation behind them.
+
+## Before you start
+
+Read [STANDARD.md](STANDARD.md) first. It defines the boundary between public,
+customer-consumable workloads and internal delivery material.
+
+Open an issue before starting a new application family, cluster target, or a
+change to an existing application's default behaviour. Direct pull requests are
+welcome for contained documentation fixes, manifest corrections, and validation
+improvements.
+
+## Scope
+
+- Keep application definitions and overlays reusable across customer
+  environments.
+- Keep `clusters/` limited to public target groupings.
+- Do not add credentials, private addresses, customer-specific domains, or
+  internal rollout material.
+- Update the relevant operator documentation when a user-visible workload
+  behaviour changes.
+
+## Validate the change
+
+Run the validator for every target affected by the change:
+
+```bash
+./scripts/validate.sh --strict --target onprem-smoke
+./scripts/validate.sh --strict --target onprem-stage1
+./scripts/validate.sh --strict --target burst
+./scripts/validate.sh --target onprem
+```
+
+For a narrowly scoped change, run at least the affected target. Run the full
+set when changing shared applications, validation logic, or cluster wiring.
+
+## Pull requests
+
+Keep each pull request focused. Include the target affected, a short summary of
+the change, and the validation commands you ran. Call out any checks you could
+not run locally.
+
+## Security
+
+Do not include secrets or vulnerability details in a public issue or pull
+request. For a suspected security issue, use the
+[HybridOps contact form](https://hybridops.tech/contact?intent=general&source=workloads-contributing&target=Security%20report&return=https%3A%2F%2Fdocs.hybridops.tech%2Fguides%2Freference%2Fcontributing%2F).