Add note about the maint-v1.x branch

Author: fhunleth

README.md

@@ -4,6 +4,9 @@
 [![CI](https://github.com/nerves-project/nerves_system_rpi0/actions/workflows/ci.yml/badge.svg)](https://github.com/nerves-project/nerves_system_rpi0/actions/workflows/ci.yml)
 [![REUSE status](https://api.reuse.software/badge/github.com/nerves-project/nerves_system_rpi0)](https://api.reuse.software/info/github.com/nerves-project/nerves_system_rpi0)
 
+*This is the v1 maintenance branch. It is still maintained and is the
+recommended branch for most users. Active development is in the `main` branch.*
+
 This is the base Nerves System configuration for the Raspberry Pi Zero and
 Raspberry Pi Zero W. See
 [`nerves_system_rpi3a`](https://github.com/nerves-project/nerves_system_rpi3a)