Create BUILDING.md

[ci skip]

BUILDING.md

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+# Building from source
+
+If you just would like to bundle your application, it is normally not necessary that you build `linuxdeployqt` yourself. Instead, download __linuxdeployqt-x86_64.AppImage__ from the [Releases](https://github.com/probonopd/linuxdeployqt/releases) page and `chmod a+x` it. This bundle, by the way, has been generated using `linuxdeployqt` itself as part of our Travis CI continuous build pipeline.
+
+So, if you still think you would like to compile from source, here are the steps:
+
+* Get and build linuxdeployqt e.g., using Qt 5.7.0 (you could use this [Qt Creator AppImage](https://bintray.com/probono/AppImages/QtCreator#files) for this)
+
+```
+sudo apt-get -y install git g++ libgl1-mesa-dev
+git clone https://github.com/probonopd/linuxdeployqt.git
+# Then build in Qt Creator, or use
+export PATH=/tmp/.mount_QtCreator-*-x86_64/5.7/gcc_64/bin/:$PATH
+cd linuxdeployqt
+qmake linuxdeployqt.pro
+make
+```
+
+* Build and install [patchelf](https://nixos.org/patchelf.html) (a small utility to modify the dynamic linker and RPATH of ELF executables; similar to `install_name_tool` on macOS). To learn more about this, see http://blog.qt.io/blog/2011/10/28/rpath-and-runpath/
+
+```
+wget https://nixos.org/releases/patchelf/patchelf-0.9/patchelf-0.9.tar.bz2
+tar xf patchelf-0.9.tar.bz2
+( cd patchelf-0.9/ && ./configure  && make && sudo make install )
+```
+
+* Optional if you want to generate AppImages: Download [appimagetool](https://github.com/probonopd/AppImageKit/releases) and put it into your $PATH, e.g., into `/usr/local/bin`. Make sure it is renamed to `appimagetool` and is `chmod a+x`
+
+```
+sudo wget -c "https://github.com/probonopd/AppImageKit/releases/download/continuous/appimagetool-x86_64.AppImage" -O /usr/local/bin/appimagetool
+sudo chmod a+x /usr/local/bin/appimagetool
+```