Readme update

[ghostsquad]

Just stumbled across this, and I thought I'd try it out, but I had to dig into the code to find out about how to use it, and even then, I'm having trouble!

awsudo --region=us-east-1 \
          arn:aws:iam::1234:role/something \
          test \
            aws sts get-caller-identity

Usage:
  awsudo [options] ROLEARN ROLESESSIONNAME [EXTERNALID]
  awsudo [options] profile PROFILENAME
  awsudo -h | --help
  awsudo -v | --version

I tried a few different combinations, but it still just outputs this help. :(