Update qt-desktop-gui.md

docs/devs/building/qt-desktop-gui.md

@@ -28,13 +28,23 @@ All commands should be run under MSYS2 mingw32.
 ```
 pacman -Suy git make tar mingw-w64-i686-gcc mingw-w64-i686-boost mingw-w64-i686-libpng mingw-w64-i686-openssl mingw-w64-i686-zlib
 pacman -Suy mingw-w64-i686-qt5-static
+pacman -Suy openssl-devel mingw-w64-i686-miniupnpc
 mkdir -p /c/dev/
 cd /c/dev/
 git clone https://github.com/PurpleI2P/i2pd.git
 cd i2pd
 git checkout openssl
+export MINGW='mingw32'
+export PATH=/$MINGW/bin:/usr/bin
+cd qt/i2pd_qt
+qmake
+make USE_UPNP=yes
 ```
 
+#### Caveats
+
+It is important to restrict PATH as described above. If you have Strawberry Perl and/or Mercurial installed, it will pick up gcc & openssl from the wrong places.
+
 TBD
 
 ## Under Ubuntu