1. Layout installation

Download layout and install it by following shell commands:

wget http://marin.jb.free.fr/qwerty-fr/manual/us_qwerty-fr
sudo cp /usr/share/X11/xkb/symbols/us /usr/share/X11/xkb/symbols/us.qwerty-fr
cat us_qwerty-fr | sudo tee -a /usr/share/X11/xkb/symbols/us_qwerty-fr

2. File configuration

a. Directory change

Change directory to /usr/share/X11/xkb/

cd /usr/share/X11/xkb/

b. File evdev.xml

Edit file rules/evdev.xml1

sudo nano rules/evdev.xml

Locate the <layout> section where configItem/name value is "us".
Then, just after <variantList> tag, add section bellow: 

<variant>
<configItem>
<name>qwerty-fr</name>
<description>French (qwerty-fr; US with french symbols)</description>
<languageList><iso639Id>fra</iso639Id></languageList>
</configItem>
</variant>

Insert should looks like something like (section inserted in red):

<layoutList>
  <layout>
    <configItem>
      <name>us</name>
      <shortDescription>USA</shortDescription>
      <description>USA</description>
      <languageList>
        <iso639Id>eng</iso639Id>
      </languageList>
    </configItem>
    <variantList>
      <variant>
        <configItem>
          <name>qwerty-fr</name>
          <description>French (qwerty-fr; US with french symbols)</description>
          <languageList>
            <iso639Id>fra</iso639Id>
          </languageList>
        </configItem>
      </variant>
      <variant>
        <configItem>
          <name>chr</name>
          <description>Cherokee</description>
          <languageList>
            <iso639Id>chr</iso639Id>
          </languageList>
        </configItem>
      </variant>
Save and exit editor.

1Conventionally, you would edit evdev.extra.xml, but due to bug #1029979, you have to update evdev.xml anyway.

c. File base(.extra).xml

You will have now to update either rules/base.extras.xml or - if this file does not exists - rules/base.xml or - if both previous files do not found - base.xml.
First verify which file exists:

ls rules/base.extras.xml rules/base.xml base.xml

And then edit accordingly the correct file:
sudo nano rules/base.extras.xml
or 
sudo nano rules/base.xml
or 
sudo nano base.xml

Make the same insertion than on file evdev.xml

Save and exit editor

d. Files base.lst and evdev.lst

Edit file rules/base.lst

sudo nano rules/evdev.lst
Locate the line containing ! variant and insert the line bellow just after
  qwerty-fr       us: qwerty-fr

Insert should looks like something like (section inserted in red): 
! variant
  qwerty-fr       us: qwerty-fr
  chr             us: Cherokee
Save and exit editor

Repeat the procedure with file rules/evdev.lst