docs: add CONTRIBUTING guide

Document the process for adding or modifying a skill, frontmatter requirements, style (British English), commit conventions, and the licence under which contributions are accepted.

CONTRIBUTING.md

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+# Contributing
+
+Guidelines for adding or modifying a skill in this catalogue.
+
+## Adding a Skill
+
+1. Pick a short, kebab-case name (`my-skill`).
+2. Create a directory and a `SKILL.md` inside it:
+   ```
+   mkdir my-skill
+   ```
+3. Start `SKILL.md` with the required frontmatter:
+   ```yaml
+   ---
+   name: my-skill
+   description: ...
+   ---
+   ```
+4. Open a pull request against `main`.
+
+### Frontmatter
+
+- `name` must match the directory name. Claude Code uses it to identify the skill.
+- `description` controls when the skill is invoked. Be specific: name the file types, project shapes, and user requests that should trigger it. A vague description (for example, "helps with Java") will fire too often and add noise.
+- The body of `SKILL.md` is loaded into the conversation when the skill fires. Keep it dense and self-contained.
+
+## Modifying a Skill
+
+Edit the skill's `SKILL.md` in place. Open a PR. If the change alters the skill's public contract (the `description`, the rules, or the inputs/outputs the skill expects), call it out in the PR description so reviewers can decide whether the change should also trigger a version bump in any catalogue that consumes it.
+
+## Style
+
+- **British English** is the default for all prose, code comments, and commit messages. Use the `british-english` skill to self-review before requesting review.
+- Use sentence case in headings ("Adding a skill", not "Adding A Skill").
+- Code samples should be runnable, or marked clearly when truncated for brevity.
+
+## Commit Messages
+
+Use conventional commits. For this repository:
+
+| Type | When |
+|---|---|
+| `feat` | A new skill, or a new feature within an existing skill |
+| `fix` | A behavioural correction |
+| `docs` | README, CONTRIBUTING, LICENCE, or doc-only edits |
+| `chore` | Tooling, `.gitignore`, build configuration |
+
+See the `commit-message` skill for the full format.
+
+## Review
+
+Reviewers check for:
+
+- Frontmatter correctness and specificity
+- Compliance with any cross-cutting rules (for example, the four-layer architecture in the `spring-boot` skill)
+- British English conformance
+- A concrete example for every non-trivial claim
+
+## Licence
+
+By contributing, you agree that your contributions are licensed under the MIT Licence (see [`LICENCE`](LICENCE)).