added contributing.md to valkey helm chart

[Bloodraven21]

Add CONTRIBUTING.md for the Valkey Helm chart

Description

The repository currently has no dedicated contributor guide, so new community members have no documented path for setting up their environment, making changes, testing, and submitting contributions. This PR adds a CONTRIBUTING.md that documents the full
contribution workflow, tailored to how this repo actually works (Justfile tasks, helm-unittest, chart-testing version enforcement, and the automated release pipeline).

What's included

• Getting Started — repository/chart overview, required tools and versions, forking & cloning with an upstream remote, and local environment setup.
• Development Workflow — feature branches, where changes live (templates/, values.yaml, values.schema.json, README.md, tests/), SemVer-based chart version bumps (and why CI requires them), and coding/documentation conventions.
• Testing — helm lint, helm template (including multi-version rendering matching CI: 1.28.15, 1.35.3, 1.36.0), helm unittest, the just shortcuts, and local end-to-end verification.
• Submitting Changes — commit message guidelines, DCO sign-off requirements (git commit -s), opening a PR with a checklist, and addressing review feedback.
• Additional Information — repository structure, the automated chart-releaser/GHCR release process, and links to Valkey, Helm, helm-unittest, the DCO, and the project Slack channel.

Why this approach

The guide references the project's real conventions rather than generic boilerplate:

• Documents the Justfile recipes (lint, test, template, template-auth, validate).
• Calls out the check-version-increment enforcement from .github/ct.yml so contributors know a chart change without a Chart.yaml bump will fail CI.
• Describes the existing automated release flow triggered by Chart.yaml version changes.

Notes for reviewers

• The file is placed at valkey/CONTRIBUTING.md (alongside the chart). If you'd prefer it at the repository root so GitHub auto-surfaces it on issues/PRs, I'm happy to move it and generalize the chart-specific sections to cover valkey-operator as well.
• Docs-only change — no chart templates or values were modified.

Acceptance criteria

• A CONTRIBUTING.md is added to the repository.
• Provides clear, step-by-step instructions for contributors.
• Documents testing, DCO sign-off, and pull request expectations.
• A new contributor can follow the guide to submit a PR end to end.

[Bloodraven21]

#184