Maintainer Docu: Git Subtree

[ax3l]

From #717

• a script how to update the thirdParty/ directory with the current updates from the according repositories (and update cmake-modules after Propose License Change to ISC cmake-modules#1 was merged)
• documentation in the maintainer wiki how to update individual thirdParty/ software via git subtree merge/pull ... --squash
• take a common third-party "author" name for imports via
  GIT_AUTHOR_NAME="Third Party" GIT_AUTHOR_EMAIL="picongpu@hzdr.de" git ...