Make the README CI badge only show the status of pushes to master

Author: penguin359

README.md

@@ -8,7 +8,7 @@ messages.
 
 APRSdroid is Open Source Software written in Scala and licensed under the GPLv2.
 
-[![Android CI](../../actions/workflows/android.yml/badge.svg)](../../actions/workflows/android.yml)
+master: [![Android CI](../../actions/workflows/android.yml/badge.svg?branch=master&event=push)](../../actions/workflows/android.yml)
 
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