android: how to add a button with text inside collapsing toolbar

You can add button and image like this

<android.support.design.widget.AppBarLayout
    android:id="@+id/appBarLayout"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:fitsSystemWindows="true">

    <android.support.design.widget.CollapsingToolbarLayout
        android:id="@+id/collapsing_toolbar"
        android:layout_width="match_parent"
        android:layout_height="150dip"
        android:fitsSystemWindows="true"
        app:contentScrim="?attr/colorPrimary"
        app:expandedTitleMarginBottom="20dp"
        app:expandedTitleMarginEnd="64dp"
        app:expandedTitleMarginStart="48dp"
        app:layout_scrollFlags="scroll|exitUntilCollapsed">

        <ImageView
            android:id="@+id/header"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            app:layout_collapseMode="parallax" />

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            app:layout_collapseMode="pin"
            app:popupTheme="@style/ThemeOverlay.AppCompat.Light" />

        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:gravity="bottom|end"
            app:layout_collapseMode="parallax"
            android:orientation="horizontal">
            <Button
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Button"/>
        </LinearLayout>
    </android.support.design.widget.CollapsingToolbarLayout>

</android.support.design.widget.AppBarLayout>

in your main scrollable content put this code

   <android.support.v4.widget.NestedScrollView
    android:id="@+id/scroll"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:clipToPadding="false"
    app:layout_behavior="@string/appbar_scrolling_view_behavior">

</android.support.v4.widget.NestedScrollView>