Document cmake-format usage in Formatting.md file

[tristan0x]

There should be a section dedicated to the formatting of CMake files to document the main features of CMake format:

• how to locally disable the formatting in a CMake file
• how to use the additional_commands in the configuration file to indicate cmake_format how to format custom functions.

[alkino]

Maybe only a link to the main cmake-format README is enough.
It's pretty clear, and this way we are still up-to-date.