Updating the README.md to point to the file USING_COLLABORATING.md (renamed...

Updating the README.md to point to the file USING_COLLABORATING.md (renamed and adapted from COLLABORATION_AGREEMENT.md). Add a file in the FLUKA directory pointing to their website for license, mentioning the desired reference and the request to register in case people are using a version with binary enclosed. Changes from GPL to BSD in README.md and USING_COLLABORATING.md to be made in another merge request.