Document Usecase for NoControlMasterMkdir=false / Default: NoControlMasterMkdir=true?

[skruppy]

For now, I don't see the use case for ControlMaster directory creation. Where does it have an advantage over:

defaults:
    NoControlMasterMkdir: true
    ControlPath: ~/.ssh/cm/%C.sock
    ControlMaster: auto

As @stk0vrfl0w already mentioned in #124, the default settings polluted my filesystem with empty directories. As it it not obvious where those directories come from, I would suggest that NoControlMasterMkdir is set to true by default (or an ControlMasterMkdir option is set to false by default). Second, it should be documented why there is an option to create those directories and how they are created.