Update README with crystal clear example of args $1 through $5

[ahw]

From hooks.vim:

let arg1 = shellescape(a:originalBufferName)
let arg2 = shellescape(a:vimHook.event)
let arg3 = shellescape(fullPathWithoutExtension)
let arg4 = shellescape(a:filenameHead)
let arg5 = shellescape(tailWithoutExtension)