[docs] initial QT documentation, move Qt Creator instructions

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+This directory contains the BitcoinQT graphical user interface (GUI). It uses the cross platform framework [QT](https://www1.qt.io/developers/).
+
+The current precise version for QT 5 is specified in [qt.mk](/depends/packages/qt.mk). QT 4 is also supported (see [#8263](https://github.com/bitcoin/bitcoin/issues/8263)).
+
+## Compile and run
+
+See build instructions ([OSX](/doc/build-osx.md), [Windows](/doc/build-windows.md), [Unix](/doc/build-unix.md), etc).
+
+To run:
+
+```sh
+./src/qt/bitcoin-qt
+```
+
+## Files and directories
+
+### forms
+
+Contains [Designer UI](http://doc.qt.io/qt-5.9/designer-using-a-ui-file.html) files. They are created with [Qt Creator](#use-qt-Creator-as IDE), but can be edited using any text editor.
+
+### locale
+
+Contains translations. They are periodically updated. The process is described [here](/doc/translation_process.md).
+
+### res
+
+Resources such as the icon.
+
+### test
+
+Tests.
+
+### bitcoingui.(h/cpp)
+
+Represents the main window of the Bitcoin UI.
+
+### \*model.(h/cpp)
+
+The model. When it has a corresponding controller, it generally inherits from  [QAbstractTableModel](http://doc.qt.io/qt-5/qabstracttablemodel.html). Models that are used by controllers as helpers inherit from other QT classes like [QValidator](http://doc.qt.io/qt-5/qvalidator.html).
+
+ClientModel is used by the main application `bitcoingui` and several models like `peertablemodel`.
+
+### \*page.(h/cpp)
+
+A controller. `:NAMEpage.cpp` generally includes `:NAMEmodel.h` and `forms/:NAME.page.ui` with a similar `:NAME`.
+
+### \*dialog.(h/cpp)
+
+Various dialogs, e.g. to open a URL. Inherit from [QDialog](http://doc.qt.io/qt-4.8/qdialog.html).
+
+### paymentserver.(h/cpp)
+
+Used to process BIP21 and BIP70 (see https://github.com/bitcoin/bitcoin/pull/11622) payment URI / requests. Also handles URI based application switching (e.g. when following a bitcoin:... link from a browser).
+
+### walletview.(h/cpp)
+
+Represents the view to a single wallet.
+
+### Other .h/cpp files
+
+* UI elements like BitcoinAmountField, which inherit from QWidget.
+* `bitcoinstrings.cpp`: automatically generated
+* `bitcoinunits.(h/cpp)`: BTC / mBTC / etc handling
+* `callback.h`
+* `guiconstants.h`: UI colors, app name, etc
+* `guiutil.h`: several helper functions
+* `macdockiconhandler.(h/cpp)`
+* `macdockiconhandler.(h/cpp)`: display notifications in OSX
+
+## Contribute
+
+See [CONTRIBUTING.md](/CONTRIBUTING.md) for general guidelines. Specifically for QT:
+
+* don't change `local/bitcoin_en.ts`; this happens [automatically](/doc/translation_process.md#writing-code-with-translations)
+
+## Using Qt Creator as IDE
+
+You can use Qt Creator as an IDE. This is especially useful if you want to change
+the UI layout.
+
+Download and install the community edition of [Qt Creator](https://www.qt.io/download/).
+Uncheck everything except Qt Creator during the installation process.
+
+Instructions for OSX:
+
+1. Make sure you installed everything through Homebrew mentioned in the [OSX build instructions](/docs/build-osx.md)
+2. Use `./configure` with the `--enable-debug` flag
+3. In Qt Creator do "New Project" -> Import Project -> Import Existing Project
+4. Enter "bitcoin-qt" as project name, enter src/qt as location
+5. Leave the file selection as it is
+6. Confirm the "summary page"
+7. In the "Projects" tab select "Manage Kits..."
+8. Select the default "Desktop" kit and select "Clang (x86 64bit in /usr/bin)" as compiler
+9. Select LLDB as debugger (you might need to set the path to your installation)
+10. Start debugging with Qt Creator (you might need to the executable to "bitcoin-qt" under "Run", which is where you can also add command line arguments)