Update CONTRIBUTORS file with external cotntributors

[d-gs]

Maintenance task

As we have already externals contributors changes on the code, they should be added to the CONTRIBUTORS.txt file (as individuals or company, depending on how they signed the CLA).

[d-gs]

@holzerma - could you have a look into this? In the PR, we will need the feedback from the contributors to confirm their name/company, and if they are fine with the inclusion on the CONTRIBUTORS.txt file.