version number in Readme.md

Hi,

Thank you for providing us such a useful extension for Slicer! Sounds exciting.

In Getting Started chapter of README.md, I am wondering if the exact version number that the extension required should be shown, because the stable and the preview version of Slicer could be updated sometimes. Or the extension always requires the preview version? I am little confused which version should I use?

And anyhow, I cannot find the extension in Extension Manager of Slicer app, if not overlooked.

Start with the Getting Started chapter in README.md and verify which Slicer version the extension requires and whether it should appear in Slicer's Extension Manager. Done means the README clearly states the required version and explains the extension's availability there.

Written by the indexing model from the issue text.