More info on NimMain procedure

[PMunch]

Just to add, it is also connected to the `--noMain` flag you pass to the 
_[compiler](https://forum.nim-lang.org/postActivity.xml#compiler). That flag 
basically means "don't automatically call the `NimMain` procedure", so often 
when you see someone calling `NimMain` they are also compiling using `--noMain`.