Add CONTRIBUTORS.txt and information from external contributors

[d-gs]

Maintenance task

Similar to LicenseScout's CONTRIBUTORS file, we should add the same file to MergeProcessor.

In addition, as we have already externals contributors changes on the code, they should be added (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.