Add introduction video to README

[Copilot]

Overview

This PR adds the project's introduction video to the README, making it prominently visible to users visiting the repository.

Changes

Added a new "Introduction Video" section to the README.md file, positioned right after the documentation link and before the "Usage" section. The video is presented as a clickable YouTube thumbnail that links to https://www.youtube.com/watch?v=Brfavid_fxw.

Motivation

The project has a wonderful introduction video that provides an excellent overview of django-tasks-scheduler and its features. However, it wasn't immediately discoverable in the project documentation. By adding it to the README with a prominent placement and clickable thumbnail, new users can quickly learn about the project through this visual resource.

Implementation Details

• Uses standard markdown syntax with YouTube's thumbnail API (https://img.youtube.com/vi/Brfavid_fxw/0.jpg)
• Positioned strategically near the top of the README for maximum visibility
• Includes descriptive text to encourage users to watch the video

Closes #[issue_number]

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• img.youtube.com
  • Triggering command: curl -s -o /tmp/youtube_thumb.jpg -w %{http_code} REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add project overview video to the README</issue_title>
<issue_description>The project has this wonderful introduction video, but it's not immediately visible in the project.</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #307

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