Add README.md in .gitlab directory

Proposal

GitLab, GitHub and BitBucket all support different extensions to the markdown syntax. I would like to create a different README.md for the diverse code forges, and keep the root README.md in strict markdown (without embedded HTML for example). This is already possible for GitHub using their .github directory, and we have a similar directory in .gitlab for issue templates, codeowners and many other files. It would make sense to check if there is a README.md in .gitlab, and if it is present, use it instead of the one in the root directory.