Document that issues should/may open in root-project/root and PR should be open in root-project/root in general

[SimeonEhrig]

I really often read, that it is better to open a issue in root-project/root. So this should be documented. If it makes sense also disable issues for root-project/cling.

PR's should be disabled in general, because the workflow, first open a PR in root-project/root, merge it and it will be automatically back ported. This needs also be documented. At the moment the PR workflow is insider knowledge.