updated readme to include options sections and example

[rob-moore]

I made the readme a little more verbose when it comes to specifying all the options that are available to purifycss.

I took the verbiage from the purifycss readme and updated the example usage in the readme.

[albertvolkman]

Are you able to get the options to work? I'm doing this and the file does not get minimized-

.pipe(purify(['./index.html']), {
  minify: true,
})

[craftedsystems]

@albertvolkman you need to place the options object inside the round brackets.

.pipe(purify(['./index.html'], {    
  minify: true,     
}))