Add example in the README / docs that uses `sort_by`

[simonprickett]

It's possible to sort results in ASCending or DESCending order like this... ASCending:

result_set = Book.find(
    (Book.author == "Robert A. Heinlein") & (Book.year_published > 1958) & (Book.year_published < 1974)
).sort_by("year_published")

and DESCending:

result_set = Book.find(
    (Book.author == "Robert A. Heinlein") & (Book.year_published > 1958) & (Book.year_published < 1974)
).sort_by("-year_published")

but there's no documentation or example for this.