Update Pi guide: systemd advice (#2181)

docs/the_nimbus_book/src/pi-guide-mainnet.md

@@ -7,3 +7,6 @@ The best thing you can do is to set your Pi to run Pyrmont. **If you have no tro
 <blockquote class="twitter-tweet" data-conversation="none"><p lang="en" dir="ltr">We&#39;ve been running lots of PIs and NanoPCs 24/7 for 3 years and never got a hardware fail. It is easy (and cheap) to get redundancy of components (even spare PIs in different locations, more of this to come).</p>&mdash; Ethereum on ARM (@EthereumOnARM) <a href="https://twitter.com/EthereumOnARM/status/1332772217420177408?ref_src=twsrc%5Etfw">November 28, 2020</a></blockquote> <script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>
 
 Although we don't expect a modern Pi to fail, we recommend buying a spare Pi, and enterprise grade SSD, on the off-chance it does; keep your original SD around, to make it easy for you to copy the image over.
+
+Finally in order to make sure your Pi autorestarts on boot, we recommend [setting up a systemd service](https://www.raspberrypi.org/documentation/linux/usage/systemd.md). For the details on how to do this, see [this page](./beacon-node-systemd.md)
+

docs/the_nimbus_book/src/pi-guide.md

@@ -408,3 +408,9 @@ To detach your `screen` session but leave your processes running, press `Ctrl-A`
 
 Verifying your progress is as simple as `ssh`ing back into your Pi and typing `screen -r`. This will resume your screen session (and you will be able to see your node's entire output since you logged out).
 
+### Professional setup advice
+
+While `screen` is a nice tool for testing, it's not really a good idea to rely on it for serious use. For a more professional setup, we recommend [setting up a systemd service](https://www.raspberrypi.org/documentation/linux/usage/systemd.md) with an autorestart on boot (should you experience an unexpected power outage, this will ensure your validator restarts correctly). 
+
+For the details on how to do this, see [this page](./beacon-node-systemd.md)
+