docs: Add Readme

packages/mermaid-layout-elk/README.md

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+# @mermaid-js/layout-elk
+
+This package provides a layout engine for Mermaid based on the [ELK](https://www.eclipse.org/elk/) layout engine.
+
+> [!NOTE]  
+> The ELK Layout engine will not be available in all providers that support mermaid by default.
+> The websites will have to install the `@mermaid-js/layout-elk` package to use the ELK layout engine.
+
+## Usage
+
+```
+flowchart-elk TD
+	A --> B
+	A --> C
+```
+
+```
+---
+config:
+	layout: elk
+---
+
+flowchart TD
+	A --> B
+	A --> C
+```
+
+```
+---
+config:
+	layout: elk.stress
+---
+
+flowchart TD
+	A --> B
+	A --> C
+```
+
+### With bundlers
+
+```sh
+npm install @mermaid-js/layout-elk
+```
+
+```ts
+import mermaid from 'mermaid';
+import elkLayouts from '@mermaid-js/layout-elk';
+
+mermaid.registerLayoutLoaders(elkLayouts);
+```
+
+### With CDN
+
+```html
+<script type="module">
+  import mermaid from 'https://cdn.jsdelivr.net/npm/mermaid@11/dist/mermaid.esm.min.mjs';
+  import elkLayouts from 'https://cdn.jsdelivr.net/npm/@mermaid-js/layout-elk@11/dist/mermaid-layout-elk.esm.min.mjs';
+
+  mermaid.registerLayoutLoaders(elkLayouts);
+</script>
+```
+
+## Supported layouts
+
+- `elk`: The default layout, which is `elk.layered`.
+- `elk.layered`: Layered layout
+- `elk.stress`: Stress layout
+- `elk.force`: Force layout
+- `elk.mrtree`: Multi-root tree layout
+- `elk.sporeOverlap`: Spore overlap layout
+
+<!-- TODO: Add images for these layouts, as GitHub doesn't support natively -->