README: add CI shields

Add shields for indicating latest example-application CI status.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>

README.md

@@ -1,5 +1,11 @@
 # Zephyr Example Application
-
+
+<a href="https://github.com/zephyrproject-rtos/example-application/actions/workflows/build.yml?query=branch%3Amain">
+  <img src="https://github.com/zephyrproject-rtos/example-application/actions/workflows/build.yml/badge.svg?event=push">
+</a>
+<a href="https://github.com/zephyrproject-rtos/example-application/actions/workflows/docs.yml?query=branch%3Amain">
+  <img src="https://github.com/zephyrproject-rtos/example-application/actions/workflows/docs.yml/badge.svg?event=push">
+</a>
 <a href="https://zephyrproject-rtos.github.io/example-application">
   <img alt="Documentation" src="https://img.shields.io/badge/documentation-3D578C?logo=sphinx&logoColor=white">
 </a>