Update README.md

[Dexdd]

Update CMake installation required.

[mithro]

Hi,

Thank you for sending the pull request! It's awesome to see you contributing to this project. However, before I can look at this request you need to add DCO sign-off to commits in this repository.

We require DCO sign-off for all commits to this repository (including all authors). Please see the following URL to see more information and BeeWare also have a Beginners guide to DCOs.

You can use git commit --amend --signoff and then a git push --force to add the signoff. You can also use git rebase --signoff.

Thanks!

[mithro]

@kgugala