Add READMEs for some functional-samples

functional-samples/sample.water_alarm_notification/README.md

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+# Drink Water Event Popup
+
+This sample demonstrates the usage and features of the event page by reminding user to drink water.
+
+## Overview
+
+This sample uses the [chrome.alarms](https://developer.chrome.com/docs/extensions/reference/api/alarms) and [chrome.notifications](https://developer.chrome.com/docs/extensions/reference/api/notifications) APIs.
+
+## 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. Select and open the extension from the extensions menu.
+4. To test the extension, click on 'Sample minute' to set an alarm of 1 minute, post which a notification appears in the system tray.

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

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+# Focus Mode
+
+This sample demonstrates [Injecting scripts into the active tab](https://developer.chrome.com/docs/extensions/get-started/tutorial/scripts-activetab).
+See [this extension based on it](https://github.com/GoogleChrome/chrome-extensions-samples/tree/main/functional-samples/tutorial.focus-mode-debugging) for more information.

functional-samples/tutorial.hello-world/README.md

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+# Hello World extension
+
+This sample demonstrates a basic structure and implementation for an extension. See [Hello World extension docs](https://developer.chrome.com/docs/extensions/get-started/tutorial/hello-world).
+
+## 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. Open the extension popup by clicking on its icon. You will see a popup that reads "Hello Extensions".

functional-samples/tutorial.reading-time/README.md

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+# Reading time
+
+Extension from the [Run scripts on every page](https://developer.chrome.com/docs/extensions/get-started/tutorial/scripts-on-every-tab) 'Get Started' article on the extensions docs.
+
+This extension adds reading time information under an article in the documentation for extensions or webstore.
+
+This sample demonstrates the use of content scripts to modify the content of a page. The content script is associated with a particular url pattern listed in `manifest.json`. The script only acts on websites whose url matches the specified pattern. In this case, the pattern matches for docs articles of extensions or webstore.
+
+## 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. Visit an article on the docs for extensions or the webstore ([Extensions Docs](https://developer.chrome.com/docs/extensions/) or [Chrome Webstore Docs](https://developer.chrome.com/docs/webstore/)).
+4. Find the Reading time information right under the article title (ensure that the extension is turned on).