Readme: Explicit optionalConfig json object options

[jo-chemla]

Simply added this section to the readme within a details note:

optionalConfig = {
  debug: true, 
  // User content
  keepnames: true, 
  keepgroups: true, 
  meta: true, 
  header: 'Header',
  // User content
  types: true, 
  shadows: true,
  exportdefault: true, 
  precision: 3,
  instance: false,
  instanceall: false
  // draco 
}

[jo-chemla]

Hi @drcmda interested in merging this micro addition to the readme or should I close the PR? Thanks!