Add small section on config backup file structure

[phillxnet]

Thanks to forum member mattyvau for inspiring this issue. It would be good to have a short doc section of our back-up config file format. Along with the neccessary formatting element that ensure it's compatibility. This way folks can be more informed if they choose too, or have a need to hand edit these files. It would also be good to have some basic guidance on how it can be made human readable.

Forum thread reference:
https://forum.rockstor.com/t/3-9-to-4-1-update-report/8249

[phillxnet]

Closing as
superseded by #465