[DOCS] You can now send to self

[darkporpoise]

Signal (and signal-cli) now allow sending messages to yourself which appear as "Note to Self" in the app.

The process for me was:

1. Android app already registered with my number on my phone (i.e. master device)
2. Install signal-cli on OctoPrint. But do NOT "register"
3. Run signal-cli link -n "OctoPrint" on Octoprint to generate a tsdevice link
4. Copy the tsdevice link to generate a QR code.
5. Back in the app on your phone Settings --> Linked devices --> +
6. Scan QR code and accept link
7. Send yourself a message with signal-cli signal-cli -u +447555000555 send -m "My first message from the CLI" +447555000555
8. Profit

[darkporpoise]

Major downside is that you don't get a notification on the app as it comes through as Note-to-self.

[aerickson]

Thanks! I'll get this into the docs.

[aerickson]

Fixed by #27.