doc: add shared-components readme (#31725)

packages/shared-components/README.md

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+# @element-hq/web-shared-components
+
+Shared React components library for Element Web, Aurora, Element modules... This package provides opinionated UI components built on top of the [Compound Design System](https://compound.element.io) and [Compound Web](https://github.com/element-hq/compound-web). This is not a design system by itself, but rather a set of big chunks of components.
+
+## Installation
+
+```bash
+yarn add @element-hq/web-shared-components
+```
+
+### Peer Dependencies
+
+This package requires compound web to be added as a peer dependency:
+
+```bash
+yarn add @vector-im/compound-web
+```
+
+## Usage
+
+### Basic Import
+
+Both JavaScript and CSS can be imported as follows:
+
+```javascript
+import { RoomListHeaderView, useViewModel } from "@element-hq/web-shared-components";
+import "@element-hq/web-shared-components/dist/element-web-shared-components.css";
+```
+
+or in CSS file:
+
+```css
+@import url("@element-hq/web-shared-components");
+```
+
+### Using Components
+
+There is two kinds of components in this library:
+
+- _regular_ react component which doesn't follow specific pattern.
+- _view_ component(MVVM pattern).
+
+> [!TIP]
+> Theses components are available in the project storybook.
+
+#### Regular Components
+
+These components can be used directly by passing props. Example:
+
+```tsx
+import { Flex } from "@element-hq/web-shared-components";
+function MyApp() {
+    return <Flex align="center" />;
+}
+```
+
+#### View (MVVM) Components
+
+These components follow the [MVVM pattern](../../docs/MVVM.md). A ViewModel instance should be provided as a prop.
+
+Here's a basic example:
+
+```jsx
+import { ViewExample } from "@element-hq/web-shared-components";
+
+function MyApp() {
+    const viewModel = new ViewModelExample();
+    return <ViewExample vm={viewModel} />;
+}
+```
+
+### Utilities
+
+#### Internationalization
+
+- `useI18n()` - Hook for translations
+- `I18nApi` - Internationalization API utilities
+
+#### Date & Time
+
+- `DateUtils` - Date formatting and manipulation
+- `humanize` - Human-readable time formatting
+
+#### Formatting
+
+- `FormattingUtils` - Text and data formatting utilities
+- `numbers` - Number formatting utilities
+
+## Development
+
+### Prerequisites
+
+- Node.js >= 20.0.0
+- Yarn 1.22.22+
+
+### Setup
+
+```bash
+# Install dependencies
+yarn install
+
+# Build the library
+yarn prepare
+```
+
+### Running Storybook
+
+```bash
+yarn storybook
+```
+
+### Write components
+
+Most the components should be written as [MVVM pattern](../../docs/MVVM.md) view components. See existing components for examples. The exception are low level components that don't need a view model.
+
+### Tests
+
+Two types of tests are available: unit tests and visual regression tests.
+
+### Unit Tests
+
+These tests cover the logic of the components and utilities. Built with Jest and React Testing Library.
+
+```bash
+yarn test
+```
+
+### Visual Regression Tests
+
+These tests ensure the UI components render correctly. They need Storybook to be running and they will run in docker using [Playwright](../../playwright.md).
+
+First run storybook:
+
+```bash
+yarn storybook
+```
+
+Then, in another terminal, run:
+
+```bash
+yarn test:storybook:update
+```
+
+Each story will be rendered and a screenshot will be taken and compared to the existing baseline. If there are visual changes or AXE violation, the test will fail.
+
+### Translations
+
+First see our [translation guide](../../docs/translation.md) and [translation dev guide](../../docs/translation-dev.md).
+To generate translation strings for this package, run:
+
+```bash
+yarn i18n
+```