Windows build: added instructions in the readme,md for building with MinGW64 tool-chain (experimental)

Readme.md

@@ -303,6 +303,27 @@ Then comes the tedious part of packaging everything in a single place so that yo
 
 You will need to install Zadig to get USB support for hardware devices. Please refer to [Zadig website](http://zadig.akeo.ie/) for details. Basically if you get things working for SDR# or HDSDR then it will work with SDRangel.
 
+<h3>MinGW64 tool-chain</h3>
+
+It is possible to use a MinGW64 tool-chain by following these steps:
+
+  - Install MSys2 from [this page](http://msys2.github.io/). Follow all the steps.
+  - Install Qt5 from MSys2 command line:
+    - `pacman -Sy  mingw-w64-x86_64-qt5`
+  - Install gcc/g++ from MSys2 command line:
+    - `pacman -Sy mingw64/mingw-w64-x86_64-gcc`
+  - Create a new "kit" in Qt Creator:
+    - Go to "Projects" sub-menu from the left menu bar
+    - Click on "Manage kits"
+    - In "Compilers" tab add a compiler naming it "MinGW64" for example. In the compiler path specify the path to `g++` in your MSys2 installation (ex: `D:\msys64\mingw64\bin\g++.exe`)
+    - In "Qt versions" tab add a Qt version and specify the path to the `qmake.exe` in your MSys2 installation (ex: `D:\msys64\mingw64\bin\qmake.exe`)
+    - In "Kits" tab add a new kit and name it "MinGW64" for example.
+      - In "Compiler" select the "MinGW64" compiler you created previously
+      - In "Qt version" select the Qt version you created previously
+    - You should now be able to use this "kit" for your build
+    - In the "Build steps" section add `CONFIG+=MINGW64` in the "Additional arguments"
+
+The final packaging in a similar way to the MinGW32 build does not seem to work. Presently it is only possible to run it from Qt Creator copying the .dll files close to the `sdrangel.exe` file in the build tree. Therefore this build is considered experimental and not provided in the releases.
 
 <h1>Android</h1>