[Doc]: add Star History add-on to README.md.

[didier-durand]

Purpose

Add a chart showing the progress of stars for the repo

Test Plan

It should render like this

for all details, see https://www.star-history.com/#vllm-project/vllm&type=date&legend=top-left

Test Result

N/A

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• [N/A] The test plan, such as providing test command.
• [N/A] The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• [N/A] (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[mergify]

Documentation preview: https://vllm--29857.org.readthedocs.build/en/29857/

[gemini-code-assist]

Code Review

This pull request adds a GitHub Star History chart to the README.md file. The change is straightforward and correctly implemented using Markdown to display a linked image from the star-history.com service. The new section is placed appropriately at the end of the file. The implementation is correct, and I have no suggestions for improvement.