title: how to randomly assign all observations to two groups evenly
body: <p>For example, my data has 2000 observations.
  I want to create a new variable called group, which essentially is a random assignment of 1 or 2 but has 1000 counts of 1 and 1000 counts of 2.</p>
  <p>Is there a command from dplyr that can do this?</p>