Add the unit times are returned in to the README

[KJTsanaktsidis]

Hey, thanks for the great library but I just spent 15 minutes combing through the source trying to work out what unit the returned times are in. I worked out that they're in seconds now, but it'd be great if this was in the README.

[copiousfreetime]

good suggestion, feel free to open a submit a pull request if you like. I'll get around to it in the next week or so if not.