WIP INSTALL.md with build instructions

INSTALL.md

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+# Development
+This guide will help you setup a development environment to launch and contribute to Meshroom.
+
+## Requirements
+### Python 2.7
+### Qt 5.10
+### Pyside2
+#### Build Instructions (based on https://fredrikaverpil.github.io/2016/08/17/compiling-pyside2/)
+
+Since PySide2 is (not yet) available in Qt installer and no prebuilt binaries are available for Qt 5.10, 
+you'll need to build it from source. The build procedure is quite similar on all platforms.
+
+Pre-requisites:
+* Python 2.7
+* Qt 5.10 (http://download.qt.io/official_releases/online_installers/)
+* llvm/libclang (http://download.qt.io/development_releases/prebuilt/libclang/)
+* CMake (https://cmake.org/download/)
+
+##### Procedure
+This procedure directly installs PySide as a package of the Python used for calling the setup script.
+Make sure to use virtualenv if you want to keep this in a separate Python environment.
+
+To simplify the build process, you can add 'bin' folders of cmake, llvm and qt in your PATH.
+Alternatively, paths to binaries can be explicitely set when calling the setup.py script (as shown below).
+
+```
+# With {libclang}/bin in PATH
+# /!\ branch 5.10 does not exist yet, but branch 5.9 is compatible with Qt-5.10
+git clone --recursive --branch 5.9 https://codereview.qt-project.org/pyside/pyside-setup 
+
+cd pyside-setup
+python setup.py --ignore-git install --cmake=/path/to/cmake --qmake=/path/to/qmake
+```
+
+