docs: explain how to point the Playground at the local build

This should help anyone looking to test their changes they made to LiquidJS via the Playground site also provided in the repository.

I have also added basic information on where the Playground is and how to run it in general.

CONTRIBUTING.md

@@ -29,3 +29,36 @@
   - `git commit -m "feat: Adding my change"`
   - `git push`
   - Go to GitHub and find your fork, open a PR against the upstream from it
+
+## Playground
+
+The Playground runs off the `docs` directory.
+`npm run:docs` is used to build it and that's included in `npm run build`.
+
+To start the site locally, go to `docs` and run `npm start`, then visit
+http://localhost:4000/playground.html.
+
+At the moment, the Playground uses the latest NPM version of the LiquidJS
+library instead of using the built artifact produced by `npm run build:dist`
+(also included in `npm run build`).
+
+To use the Playground with the local build of the library, make these changes:
+
+- Copy `dist/liquid.browser.min.js` to `docs/public/js/liquid.browser.min.js`
+- Open `docs/themes/navy/layout/partial/after_footer.swig`
+- Remove `https://cdn.jsdelivr.net/npm/liquidjs/dist/liquid.browser.min.js` line
+- Add `{{ js('liquid.browser.min.js') }}` line before `{{ js('js/main') }}` line
+- Refresh the Playground site for the changes to take effect
+- Replace `liquid.browser.min.js` each time after making changes and building
+
+```diff
+ {% if page.layout === 'playground' %}
+-<script src="https://cdn.jsdelivr.net/npm/liquidjs/dist/liquid.browser.min.js"></script>
+ <script src="https://cdn.jsdelivr.net/npm/[email protected]/src-min/ace.js"></script>
+ {% endif %}
+ 
++{{ js('js/liquid.browser.min.js') }}
+ {{ js('js/main') }}
+ 
+ <script src="https://cdn.jsdelivr.net/npm/docsearch.js@2/dist/cdn/docsearch.min.js"></script>
+```