Add document for how to start farmbot_os in development mode

docs/DEVELOPMENT.md

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+# Starting FarmbotOS in `host` env.
+
+Certain behaviour is slightly different when developing on `host` compared to
+deploying on target environments such as `rpi` or `rpi3`.
+
+## Firmware
+By default, an Arduino does _not_ need to be connected to your `host` pc when
+doing development. See the `FarmbotFirmware.Transport` module for more info
+on this topic. If you want to use a real Arduino in `host` mode, you can
+export a `FARMBOT_TTY` environment variable in your `host` environment.
+
+```bash
+$ cd farmbot_os
+$ export FARMBOT_TTY=/dev/ttySomeDeviceFile
+$ mix compile --force
+```
+
+If you're device moves ttys, you will have to redo this step.
+
+## Configurator
+Currently configurator does not run on the `host` enviornment. To connect to
+your FarmBot account, export the following variables:
+
+```bash
+$ cd farmbot_os
+$ export FARMBOT_EMAIL=test@test.com
+$ export FARMBOT_PASSWORD=password123
+$ export FARMBOT_SERVER=http://localhost:3000
+```