add installation guidelines to readme

[MichaelChirico]

i'm a recent atom convert and so completely new to its extensions interface (in fact stata highlighting is the first i've sought out).

it would be helpful for folks like me to see a quick 1-2-3 on how to get this up and running. it looks like exactly what i'm after (i'm just reading others' stata code, not actually executing), but i'm afraid i'll end up with the stata-exec package since it has this walk-through readily available