Update README.md

[dcoeurjo]

Thanks a lot for contributing to DGtalTools, before submitting your PR, please fill up the description and make sure that all checkboxes are checked.

PR Description

your description here

Checklist

• [ ] Doxygen documentation of the code completed (classes, methods, types, members...).
• [ ] Main tool doxygen documentation (following existing documentation of DGtalTools documentation.
• [ ] Check if it follows the tools structure described in CONTRIBUTING.md
• [ ] New entry in the ChangeLog.md added.
• [ ] Update the readme with potentially a screenshot of the tools if it applies.
• [ ] No warning raised in Debug cmake mode (otherwise, Travis C.I. will fail).
• [ ] All continuous integration tests pass (Github Actions & appveyor).