readme: add badges

Add CI badges to readme.

Signed-off-by: Giacomo Dematteis <giacomo.dematteis@nordicsemi.no>

Author: DematteisGiacomo

README.md

@@ -1 +1,7 @@
 # Asset Tracker Template
+
+#### Oncommit:
+[![Target tests](https://github.com/NordicSemiconductor/Asset-Tracker-Template/actions/workflows/build-and-target-test.yml/badge.svg)](https://github.com/NordicSemiconductor/Asset-Tracker-Template/actions/workflows/build-and-target-test.yml)
+
+#### 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)