How to integrate a floating action button into linear layout with toolbar

This worked for me:

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
tools:context=".HomeFeedActivity">
<android.support.design.widget.FloatingActionButton
    android:id="@+id/fab"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="bottom|end"
    android:layout_margin="@dimen/fab_margin"
    android:src="@drawable/ic_action_add" />

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical"
    xmlns:tools="http://schemas.android.com/tools"
    android:fitsSystemWindows="true"
    tools:context=".HomeFeedActivity">

    <ListView
        android:id="@+id/list"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:divider="@null" />

</LinearLayout>

Try to put ListView and FloatingActionButton inside FrameLayout

<?xml version="1.0" encoding="utf-8"?>
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
   android:layout_width="fill_parent"
   android:layout_height="fill_parent"
   android:orientation="vertical"
   android:background="@drawable/background_serious" >

      <include layout="@layout/toolbar"/>
  <FrameLayout
         android:layout_width="match_parent"
         android:layout_height="match_parent">

          <ListView android:id="@id/android:list"
              android:layout_width="fill_parent"
              android:layout_height="wrap_content"
              android:cacheColorHint="#00000000">
          </ListView>

     <android.support.design.widget.FloatingActionButton
         android:id="@+id/fab"
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:layout_gravity="end|bottom"
         android:layout_margin="@dimen/fab_margin"
         android:src="@drawable/ic_done" />

  </FrameLayout>
</LinearLayout>

you need to use android.support.design.widget.CoordinatorLayout as your root layout instead of LinearLayout then only android.support.design.widget.FloatingActionButton will work

<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical"
    android:background="@drawable/background_serious" >

// your code

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