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

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.