Add missing readme files. Fixes #798 (#1616)

functional-samples/sample.water_alarm_notification/README.md

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+# Drink Water Alarm
+
+This example demonstrates how to use the [Alarms API](https://developer.chrome.com/docs/extensions/reference/api/alarms) and [Notifications API](https://developer.chrome.com/docs/extensions/reference/api/notifications) to remind users to drink water at set intervals.
+
+## Overview
+
+The extension allows users to set a recurring reminder to stay hydrated. Users can choose from preset intervals (1, 15, or 30 minutes) through a popup interface. When the alarm fires, a notification appears prompting the user to drink water.
+
+## Implementation Notes
+
+The extension uses several Chrome APIs together:
+
+- `chrome.alarms` - Creates and manages the reminder timer
+- `chrome.notifications` - Displays the hydration reminder when the alarm fires
+- `chrome.storage.sync` - Persists the user's selected interval across sessions
+- `chrome.action` - Shows an "ON" badge when an alarm is active
+
+When the notification appears, users can click "Keep it Flowing" to restart the timer with their previously selected interval.
+
+## Running this extension
+
+1. Clone this repository.
+2. Load this directory in Chrome as an [unpacked extension](https://developer.chrome.com/docs/extensions/mv3/getstarted/development-basics/#load-unpacked).
+3. Click the extension icon to open the popup.
+4. Select a time interval (Sample minute, 15 Minutes, or 30 Minutes).
+5. Wait for the notification to appear reminding you to drink water.

functional-samples/tutorial.focus-mode/README.md

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+# Focus Mode
+
+This example demonstrates how to use the [Scripting API](https://developer.chrome.com/docs/extensions/reference/api/scripting) to inject and remove CSS on demand, enabling a distraction-free reading experience on Chrome's documentation pages.
+
+## Overview
+
+The extension adds a focus mode toggle for Chrome's Extensions and Chrome Web Store documentation. When activated, it hides navigation elements and other distractions, centering the article content for easier reading.
+
+## Implementation Notes
+
+The extension uses the following Chrome APIs:
+
+- `chrome.scripting.insertCSS()` - Injects the focus mode stylesheet when enabled
+- `chrome.scripting.removeCSS()` - Removes the stylesheet when disabled
+- `chrome.action` - Displays an "ON"/"OFF" badge indicating the current state
+- `activeTab` permission - Allows the extension to modify only the current tab
+
+The focus mode can be toggled by:
+
+- Clicking the extension icon
+- Using the keyboard shortcut (Ctrl+B on Windows/Linux, Command+B on Mac)
+
+The toggle state is maintained per-tab, allowing different tabs to have different focus mode states.
+
+## Running this extension
+
+1. Clone this repository.
+2. Load this directory in Chrome as an [unpacked extension](https://developer.chrome.com/docs/extensions/mv3/getstarted/development-basics/#load-unpacked).
+3. Navigate to https://developer.chrome.com/docs/extensions/ or https://developer.chrome.com/docs/webstore/.
+4. Click the extension icon or press Ctrl+B (Command+B on Mac) to toggle focus mode.

functional-samples/tutorial.open-api-reference/README.md

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+# Open Extension API Reference
+
+This example demonstrates how to use the [Omnibox API](https://developer.chrome.com/docs/extensions/reference/api/omnibox) to quickly navigate to Chrome extension API reference pages directly from the address bar.
+
+## Overview
+
+The extension registers the keyword "api" in Chrome's omnibox. When users type "api" followed by a space, they can search for any Chrome extension API and navigate directly to its documentation page.
+
+## Implementation Notes
+
+The extension uses several Chrome APIs:
+
+- `chrome.omnibox` - Registers the "api" keyword and handles input/selection events
+- `chrome.storage.local` - Stores recent search history to provide personalized suggestions
+- `chrome.alarms` - Used for periodic tip fetching from an external service
+- Content script - Runs on the API reference pages
+
+When the user types in the omnibox:
+
+1. `onInputChanged` provides API suggestions based on user input and search history
+2. `onInputEntered` opens the selected API's reference page in a new tab
+3. The most recent searches (up to 4) are saved for future suggestions
+
+The service worker is organized into modules:
+
+- `sw-omnibox.js` - Handles omnibox events and navigation
+- `sw-suggestions.js` - Generates API suggestions
+- `sw-tips.js` - Fetches extension development tips
+
+## Running this extension
+
+1. Clone this repository.
+2. Load this directory in Chrome as an [unpacked extension](https://developer.chrome.com/docs/extensions/mv3/getstarted/development-basics/#load-unpacked).
+3. Type "api" in the address bar and press Space or Tab.
+4. Start typing an API name (e.g., "tabs", "storage", "scripting").
+5. Select a suggestion or press Enter to open the API reference page.