Add some context as to what are physicals

[laijasmine]

Add a blurb along the lines of this to help explain what physicals are in the set physical section. (in maybe a info box)

The physical portion describes a lot of the properties of the file so it contains a lot of information about the data format, the file size, the name of the file etc. It’s a lot of info so we usually let our helper function take care of it.

The word ‘physical’ derives from database systems, which distinguish the ‘logical’ model (e.g., what attributes are in a table, etc) from the physical model (how the data are written to a physical hard disk (basically, the serialization). so,  we grouped metadata about the file as written to disk in physical.