Expand README with site structure, local preview, and contributing notes

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

Author: aclark4life

README.rst

@@ -1,4 +1,38 @@
 Pillow Website
 ==============
 
-This website is live at `python-pillow.github.io <https://python-pillow.github.io/>`_
+This website is live at `python-pillow.github.io <https://python-pillow.github.io/>`_.
+
+It's a static site — plain HTML, CSS, and a touch of vanilla JS, with no build
+step or framework. GitHub Pages serves it directly from the ``main`` branch.
+
+Structure
+---------
+
+- ``index.html`` — the entire site (single page)
+- ``assets/css/pillow.css`` — all styles
+- ``assets/js/back-to-top.js`` — the "back to top" button behavior
+- ``assets/images/`` — logos, hero art, and other images
+- ``assets/fontawesome/`` — vendored Font Awesome icons
+
+Local preview
+-------------
+
+Since there's no build step, any static file server works, for example::
+
+    python3 -m http.server 8000
+
+Then open http://localhost:8000/ in a browser.
+
+Contributing
+------------
+
+This repo uses `pre-commit <https://pre-commit.com/>`_ for basic hygiene
+checks (trailing whitespace, end-of-file, YAML/JSON validation, etc.). Install
+it and run against all files with::
+
+    pre-commit run --all-files
+
+or, if you have `just <https://github.com/casey/just>`_ installed::
+
+    just precommit