Add CONTRIBUTING.md so AGENTS.md link resolves

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: MariusStorhaug

CONTRIBUTING.md

@@ -0,0 +1,35 @@
+# Contributing
+
+Thanks for helping improve the PSModule documentation. Everything here is a work in progress — if you find a
+problem, fix it if it's small, or open an issue if it needs more discussion.
+
+## Repository layout
+
+- `src/docs/`: the documentation source, built as a [Zensical](https://zensical.org/) site.
+- `src/includes/`, `src/overrides/`: shared snippets and theme customizations for the site.
+- `src/zensical.toml`: site configuration and navigation.
+- `guidance/`: example PowerShell snippets referenced from the docs.
+
+## Making changes
+
+1. Create a worktree/branch for your change (see [Git Worktrees](https://msxorg.github.io/docs/Ways-of-Working/Git-Worktrees/)).
+2. Edit the relevant page(s) under `src/docs/`. Update `src/zensical.toml` navigation if you add or move a page.
+3. Open a draft pull request early and push small, incremental commits so the change is easy to review.
+4. Merge to `main` once review passes. The `Docs` workflow (`.github/workflows/Docs.yml`) builds and publishes the
+   site to GitHub Pages on every merge to `main`.
+
+## Building and previewing locally
+
+```powershell
+pip install zensical
+cd src
+zensical build --clean
+```
+
+The built site is written to `src/site/`. See the [Zensical docs](https://zensical.org/docs/setup/basics/) for
+live-reloading preview options.
+
+## Linting
+
+Pull requests are linted with [super-linter](https://github.com/super-linter/super-linter) via the `Docs` workflow.
+Keep Markdown changes consistent with the surrounding style and fix any linter findings reported on the PR.