Update README.md

State that the template is still under development.

Signed-off-by: Jan Tore Guggedal <jantore.guggedal@nordicsemi.no>

Author: jtguggedal

README.md

@@ -5,3 +5,6 @@
 
 #### Nightly:
 [![Target_tests](https://github.com/NordicSemiconductor/Asset-Tracker-Template/actions/workflows/build-and-target-test.yml/badge.svg?event=schedule)](https://github.com/NordicSemiconductor/Asset-Tracker-Template/actions/workflows/build-and-target-test.yml?query=branch%3Amain+event%3Aschedule)
+
+
+The Asset Tracker Template is under development.