Added documentation on how arguments are passed by pre-commit to the hooks

Some notes on how arguments are passed to the script/command. I think this will be useful for those that need to create script or system type hooks.

I have been writing some custom hooks (for PHP) with the pre-commit framework. What I tend to struggle with was understanding how the arguments from the `args` property and the list of staged files were passed on to the hook entry for final execution.

I found the site did not mention how this was done and resulted in me having to do several tries to figure it out. 

Feel free to edit the wording if needed. I just needed this for my own reference.