Merge branch 'opentofu-note-in-readme' into 'master'

Add OpenTofu note to README

See merge request https://gitlab.com/gitlab-org/terraform-images/-/merge_requests/254



Merged-by: João Alexandre Cunha <j.a.cunha@gmail.com>
Approved-by: João Alexandre Cunha <j.a.cunha@gmail.com>
Co-authored-by: Timo Furrer <tfurrer@gitlab.com>