Document expected sort method

[rosszurowski]

I've been working with this library to implement a reorderable interface, and ran into a point of confusion.

As someone who's worked with JS a while, I avoid the default .sort() method by default, since it commonly gives incorrect results when sorting strings. My default inclination is to do something like the following:

const items = [{ id: "1", sortOrder: "a0" }, { id: "2", sortOrder: "a0l" }, { id: "3", sortOrder: "a0V" }]
const sortedItems = items.sort((a, b) => a.sortOrder.localeCompare(b.sortOrder))

Using localeCompare caused my sort order to be incorrect, which resulted in a separate reorder method throwing errors when calling generateKeyBetween.

I saw from #19 that the preferred method is .sort(), or I suppose a re-implementation of the default sort algorithm, like so:

const sortedKeys = keys.sort((a, b) => {
  if (a.sortOrder < b.sortOrder) return -1
  if (a.sortOrder > b.sortOrder) return 1
  return 0
})

It might be nice to document that in the readme somewhere, so others don't run into the same issue.