Clarify readme language on dependency direction with Dev Kit

From some feedback from non-native English speaking folks, some of the language is being misinterpreted in intro paragraph of Readme, making it seem like C# Dev Kit comes with this extension or is required.

Current language (emphasis mine)"

> A [Visual Studio Code](https://code.visualstudio.com/) [extension](https://marketplace.visualstudio.com/VSCode) that provides rich language support for C# and **is shipped along with [C# Dev Kit](https://marketplace.visualstudio.com/items?itemName=ms-dotnettools.csdevkit)**.

Suggested change by customer:

>A [Visual Studio Code](https://code.visualstudio.com/) [extension](https://marketplace.visualstudio.com/VSCode) that provides rich language support for C#. Even more language support is provided by the [C# Dev Kit](https://marketplace.visualstudio.com/items?itemName=ms-dotnettools.csdevkit) extension which can be installed additionally.

Or something along those lines that doesn't feel dependency related in the wrong direction.

Ref: https://github.com/microsoft/vscode-dotnettools/issues/403#issuecomment-1683501779