[docs] Update README.md to include extras example (#1806)

## Summary

<!-- What's the purpose of the change? What does it do, and why? -->
There was no example of to hint support for extras in the README.md.

I added one example in the install section that is used in the uv tests.

README.md

@@ -76,6 +76,7 @@ uv pip install flask                # Install Flask.
 uv pip install -r requirements.txt  # Install from a requirements.txt file.
 uv pip install -e .                 # Install the current project in editable mode.
 uv pip install "package @ ."        # Install the current project from disk
+uv pip install "flask[dotenv]"      # Install Flask with "dotenv" extra.
 ```
 
 To generate a set of locked dependencies from an input file: