Add README.md with Markdown HTML Renderer documentation

README.md

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+# Markdown HTML Renderer
+
+Small, dependency-free Markdown-to-HTML converter originally built for the
+"Heimgeist" AI chat frontend. It focuses on predictable, chat-friendly rendering
+and ships with a matching CSS theme.
+
+## What it does
+
+- Removes `<think>...</think>` and `<thinking>...</thinking>` blocks.
+- Normalizes exotic spaces to regular spaces.
+- Supports headings (`#` to `####`), horizontal rules, blockquotes, lists,
+  GitHub-style tables, inline bold/italic/code, and fenced code blocks.
+- Escapes HTML outside fenced code blocks for safety.
+- Balances unfinished fenced code blocks for streaming output.
+- Converts newlines to `<br>` and normalizes spacing around block elements.
+- Adds a code-block header with language label and a copy button.
+- Sanitizes links to allow only `http(s)`, `mailto:`, `tel:`, `/` and `#`.
+
+## Files
+
+- `markdown.js` — exports the renderer.
+- `markdown-render.css` — styles for the generated HTML.
+
+## Usage
+
+```js
+import { markdownToHTML } from './markdown.js';
+
+const markdown = `
+# Title
+
+Here is **bold**, *italic*, and \`inline code\`.
+
+> Blockquote
+
+1. One
+2. Two
+
+| Col A | Col B |
+|:-----|------:|
+| left | right |
+
+\`\`\`js
+console.log('hello');
+\`\`\`
+`;
+
+const html = markdownToHTML(markdown);
+
+// Example: render into the DOM
+document.querySelector('#output').innerHTML = html;
+```
+
+Include the CSS and optionally wrap the output:
+
+```html
+<link rel="stylesheet" href="markdown-render.css">
+<div class="md-root" id="output"></div>
+```
+
+## Copy button behavior
+
+Each fenced code block includes a button with a URL-encoded payload:
+
+```html
+<button data-copy-code="...">...</button>
+```
+
+You can wire it up like this:
+
+```js
+document.addEventListener('click', (event) => {
+  const button = event.target.closest('.md-codeblock__copy');
+  if (!button) return;
+  const raw = button.getAttribute('data-copy-code') || '';
+  const code = decodeURIComponent(raw);
+  navigator.clipboard.writeText(code);
+});
+```
+
+## Notes and limitations
+
+- This is a regex-based renderer, not a full Markdown parser.
+- Only fenced code blocks are supported (no indented code blocks).
+- Nested lists and advanced Markdown extensions are not parsed.
+- Link conversion is deliberately strict for safety.
+- The link-matching regex is tailored to the current implementation in
+  `markdown.js`; if you need standard `[label](url)` parsing, adjust it there.
+
+## License
+
+Unspecified. Add a license file if you plan to distribute this module.