update readme

Author: amazingfate

README.md

@@ -1,4 +1,4 @@
 # Armbian images for Rock5B
 [![image_build](https://github.com/amazingfate/armbian-rock5b-images/workflows/Build/badge.svg)](https://github.com/amazingfate/armbian-rock5b-images/actions/workflows/build.yml)
 
-Don't download these test images if you don't know what it is!
+This is the testing build of armbian current developing branch.