Doc: Add missing file

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Change-Id: I44246688f5d8b0baf83069475fbd3c0a5c9a4c7d
Reviewed-by: Topi Reiniö <topi.reinio@qt.io>

doc/qtcreator/src/qtquick/qtquick-placeholder-data.qdoc

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+/****************************************************************************
+**
+** Copyright (C) 2020 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the Qt Creator documentation.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Free Documentation License Usage
+** Alternatively, this file may be used under the terms of the GNU Free
+** Documentation License version 1.3 as published by the Free Software
+** Foundation and appearing in the file included in the packaging of
+** this file. Please review the following information to ensure
+** the GNU Free Documentation License version 1.3 requirements
+** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
+**
+****************************************************************************/
+
+/*!
+    \page qtquick-placeholder-data.html
+    \if defined(qtdesignstudio)
+    \previouspage studio-simulation-overview.html
+    \nextpage studio-javascript.html
+    \else
+    \previouspage qtquick-annotations.html
+    \nextpage creator-quick-ui-forms.html
+    \endif
+
+    \title Loading Placeholder Data
+
+    The Design mode supports views, models, and delegates, so that when you add
+    a Grid View, List View, or Path View item, the ListModel and the delegate
+    item are added automatically.
+
+    However, the missing context of the application presents a challenge.
+    Specific models defined in C++ are the most obvious case. Often,
+    the context is missing simple properties, which are either defined in C++,
+    or in other QML files. A typical example is an item that uses the
+    properties of its parent, such as \c parent.width.
+
+    \section1 Using Dummy Models
+
+    If you open a file in the Design mode that references a C++ model, you see
+    nothing on the canvas. If the data in the model is fetched from the
+    internet, you have no control over it. To get reliable data, \e {dummy data}
+    was introduced.
+
+    For example, the following code snippet describes the file example.qml that
+    contains a ListView that in turn specifies a C++ model:
+
+    \qml
+        ListView {
+            model: dataModel
+            delegate: ContactDelegate {
+                name: name
+            }
+        }
+    \endqml
+
+    Create a directory named \e dummydata in the root directory of the project,
+    so that it is not deployed to the device. In the \c dummydata directory,
+    create a QML file that has the same name as the value of \c model:
+
+    \code
+        qml/exampleapp/example.qml
+        dummydata/dataModel.qml
+    \endcode
+
+    Then create the dataModel.qml file that contains the dummy data:
+
+    \qml
+        import QtQuick 2.0
+
+        ListModel {
+             ListElement {
+                 name: "Ariane"
+             }
+             ListElement {
+                 name: "Bella"
+             }
+             ListElement {
+                 name: "Corinna"
+             }
+        }
+    \endqml
+
+    \section1 Creating Dummy Context
+
+    The following example presents a common pattern in QML:
+
+    \qml
+        Item {
+            width: parent.width
+            height: parent.height
+        }
+    \endqml
+
+    This works nicely for applications but the Design mode displays a zero-sized
+    item. A parent for the opened file does not exist, because the context is
+    missing. To get around the missing context, the idea of a \e {dummy
+    context} is introduced. If you place a file with the same name as the
+    application (here, example.qml) in the \c {dummydata/context} directory,
+    you can fake a parent context:
+
+    \qml
+        import QtQuick 2.0
+        import QmlDesigner 1.0
+
+        DummyContextObject {
+            parent: Item {
+                width: 640
+                height: 300
+            }
+        }
+    \endqml
+*/