Tell users to safely eject the SD card after flashing

This is here mostly so we can mention that their computer won't
be able to read the data, which would otherwise be a scary warning
from their computer.

Fixes #291

Author: PeterJCLaw

kit/brain_board/index.md

@@ -42,3 +42,7 @@ If you choose to use a tool other than Etcher, you may need to extract the `srob
 1. Open Etcher and select the `srobo-robot-{{ latest_version }}.img.xz` file you downloaded
 2. Select your SD card from the devices window
 3. Click 'Flash!'
+4. When the flash is complete you should safely eject the SD card.<br>
+   Your computer may complain that the SD card is no longer readable, however
+   this is expected as the data being written to the SD card is not in a format
+   that either Windows or macOS can natively understand.