Improve documentation for slide notes

[sumeetattree]

It took me too long to figure out that the last entry in the ppt note array is the slide number. Could you please document this?

Another suggestion I would like to make is to return the notes_content as a hash instead of an array.

{
  <slide_number>: [note_content]
}

[pythonicrubyist]

@sumeetattree I will update the docs, but I can not change the format of notes_content because of backwards-compatibility issues.