Add a basic contributing guide

CONTRIBUTING.md

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+# Contributing
+
+Contributing to the docs is pretty easy.
+
+If it's just a single typo fix or similar in just one file, you can just do it straight from the web interface or the GitHub app:
+
+1. Find the page you're looking for. Example: https://mithril.js.org/animation.html
+2. Go to the same-named path in the `docs/` directory here. Example: https://github.com/MithrilJS/docs/blob/main/docs/animation.md
+3. Click the pencil icon to edit.
+4. Make the changes you want.
+5. Click "Commit changes" and fill out that mini form. Description is optional, but helpful if you need more explanation than the title can afford.
+6. Click "Propose changes" and go through the form to create a pull request. Make sure the checkbox to allow maintainer edits is checked, so we can more easily merge it if it can't be merged cleanly.
+
+If it's something bigger, you can roughly follow [these instructions here](https://mithril.js.org/contributing.html#how-do-i-send-a-pull-request?). For the docs, there really isn't anything to test, so you can skip that step for those. Elsewhere, testing is all manual.
+
+## Scripts, or how to view the docs as you're editing them locally
+
+There's a few commands you can use to help hash out larger changes. The last command is the one you'll likely be using the most.
+
+> Note that you'll need to run `npm install` to run any of these (and obviously, you'll need to have Node installed). Also, you'll need Node v20.12.2 or later.
+
+- `npm run lint:docs`: Lint just the docs.
+- `npm run lint`: Lint the docs and scripts.
+- `npm run build`: Build the docs.
+- `npm run watch`: Build the docs on change, lint the docs on change, and start a local server to view the generated docs at `http://localhost:8080/`.