Clarify changelog fragment workflow for contributors

## Summary

I opened this as a repository documentation/file-management improvement suggestion, but the original issue body was accidentally pasted as a JSON object and included a few overbroad claims. Reframing this into a narrower, more actionable docs request.

Vault already has many of the expected repository files and docs in place:

- root `CHANGELOG.md`
- granular `changelog/` entries
- `.github/ISSUE_TEMPLATE/`
- `.github/ISSUE_TEMPLATE.md`
- `.github/pull_request_template.md`
- `.github/CODE_OF_CONDUCT.md`
- README links to docs, tutorials, forum, development setup, testing, and troubleshooting

The remaining improvement I wanted to suggest is mostly about contributor clarity.

## Problem

For a new or occasional contributor, it is not immediately obvious how the root `CHANGELOG.md` and the `changelog/` fragment directory relate to each other.

Because both exist, contributors may wonder:

- Should a PR add a new file under `changelog/`?
- Is `CHANGELOG.md` generated from fragments or manually maintained?
- What filename/category format should a changelog fragment use?
- Are there changes that should not receive a changelog entry?

This is not a request to remove either changelog mechanism. It is a request to document the intended workflow more explicitly.

## Suggested Improvements

### 1. Add changelog contributor guidance

Add a short `changelog/README.md` or a section in `CONTRIBUTING.md`/the PR template that explains:

- when to add a changelog fragment;
- expected filename format;
- expected content/category format;
- how fragments are collected into release notes or `CHANGELOG.md`;
- examples of changes that do and do not need changelog entries.

### 2. Link changelog guidance from the PR template

If Vault expects changelog fragments for user-facing changes, the PR template could include a small checklist item such as:

```md
- [ ] Added a changelog fragment, or confirmed this change does not need one.
```

### 3. Optional README contributor pointer

The README already links to user docs and developer setup. A small contributor pointer could help route people toward the right docs before opening PRs, for example:

```md
For contribution workflow, changelog entries, and PR expectations, see CONTRIBUTING.md.
```

## Non-goals

- Do not remove `CHANGELOG.md`.
- Do not remove the `changelog/` fragment directory.
- Do not replace existing GitHub issue/PR templates.
- Do not duplicate product documentation already maintained on developer.hashicorp.com.

## Why this helps

This would reduce uncertainty for contributors and make changelog expectations easier to follow without changing Vault's existing release process.

Start by comparing the root CHANGELOG.md, the changelog/ directory, and the contributor-facing files mentioned in the issue, including CONTRIBUTING.md and .github/pull_request_template.md. Document the existing fragment workflow, filename and category rules, and when entries are optional; done means contributors can follow the process without guessing and the relevant guidance is linked from the PR template.

Written by the indexing model from the issue text.