add readme

Author: litongjava

app/src/main/res/layout/activity_main.xml

@@ -46,7 +46,6 @@
     android:id="@+id/sample_text"
     android:layout_width="wrap_content"
     android:layout_height="wrap_content"
-    android:text="Hello World!"
     app:layout_constraintBottom_toBottomOf="parent"
     app:layout_constraintLeft_toLeftOf="parent"
     app:layout_constraintRight_toRightOf="parent"