message: Add Resin envvar info to compute readme
diff: * [Running Container](#runtime)
  * [Flashing a device](#flashing)
  * [Deploying a container](#deployment)
  -
  +* [Things to know](#misc)
  Welcome to the compute directory! The Opentrons API runs on a Raspberry Pi 3 running ResinOS.
  @@ -40,7 +40,7 @@ In the top-right corner of your application page, you'll find the command to add
  ## Running Container
  -There are a few small things to note about the runtime envionrment of compute device
  +There are a few small things to note about the runtime environment of compute device
  such as enviornment variables in the `OT` and `RESIN` namespaces.
  **OT environment variables**
  @@ -81,8 +81,13 @@ However, this list is subject to change. When adding remote targets, as indicate
  naming the targets according to the application they correspond to so you can run `git push devbots`.
  +## Things to know
  +Here are some things to know - many of these are resin or compute gotchas.
  -
  +**Communicating over serial**
  +Make sure that `RESIN_HOST_CONFIG_dtoverlay` is set to `pi3-miniuart-bt` for the Fleet configuration on any fleet of bots.
  +This allows the Pi to utilize the uart pins to communicate with smoothie. Without this envvar set, serial communication
  +between the pi and smoothie will not work.
  