Check Maintainers Name in Repo File

[loreanvictor]

Currently, the validate and the update repo table scripts only check the URL of the repository and not the names of the maintainers. This has already caused an issue once where maintainers have entered their name instead of their GitHub user names, which results in wrong links in the repo table (and in case there is a space inside their name, weird formatting).

Since this results in confusion and wrong data entry, I will mark it as a bug.

• Update readme, so that it is more clear that GitHub username must be entered
• Update validate script, so that it also checks URLs for maintainers and fails the CI check