Add a README for the bundle

It's the main README for building the game.

README.md

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+
+# Planet Blupi development bundle
+
+This bundle is the main repository for building the _Planet Blupi_ game. If
+you are an official distribution packager, maybe you can directly use the
+`https://github.com/blupi-games/planetblupi.git` repository instead of this
+one but **it's not the recommended way** and in the case of a distribution,
+it needs some improvements in the `CMakeLists.txt` file (based on [CMake][6]).
+
+The game is built with static linking as much as possible. The goal is to
+limit the linking of dynamic libraries which are not available natively on
+the host operating system of most users. There is only one exception on Windows
+about the use of the dynamic `libwinpthread-1.dll` library. Most dependencies
+are built via the `CMakeLists.txt` file provided here (linpng, FFmpeg, SDL2,
+and more with the appropriate flags). For example, FFmpeg is built only with
+the necessary codecs.
+
+For Linux, the release is packaged in an [AppImage][1]. The user can download
+the image and adds the executable flag. Then the game is ready to play (no root
+needed, no dependencies to install). In the case of Darwin, it's mostly like
+Linux but with a `.app` in a `DMG` image. Just open the `DMG` and the game can
+be played immediatly (the user can copy the game in the `/Applications`
+directory if he wants). Only Windows is a bit different. Even if the game is
+standalone, the installation is provided via an [NSIS][2] installer.
+
+> Note that the `DMG` bundling and the [NSIS][2] building is directly provided
+> by [CMake][6]. In the case of [AppImage][1], it's handled by [CMake][6] too
+> but via a custom `.cmake` file available in the `planetblupi/cmake` directory.
+
+## Prepare environment
+
+```sh
+git clone --recursive https://github.com/blupi-games/planetblupi-dev.git
+cd planetblupi-dev
+```
+
+The source-code is written in C++, only [GCC][4] and [Clang][5] are officially
+supported. The clone must be `recursive` because the development use git
+submodules.
+
+-   **source-code**: `planetblupi/`
+-   **source-assets**: `planetblupi-data/`
+-   **website**: `planetblupi-www/` (see <http://www.blupi.org>)
+
+### Linux
+
+You need the usual development packages for C++ ([GCC][4]) with pkg-config and
+[CMake][6].
+
+### Darwin
+
+You need the development commandline tools provided by Apple via Xcode (Clang),
+pkg-config and [CMake][6].
+
+### Windows with [MSYS2][3]
+
+In the case of Windows, it's not recommended to use MSVC because you will have
+some difficulties to have all dependencies. The best way is using [MSYS2][3]
+with it's great package manager (pacman).
+
+```sh
+# Update your toolchain
+pacman -Syuu
+
+# Install main development packages
+pacman -S --noconfirm --needed make pkg-config mingw-w64-x86_64-toolchain mingw-w64-x86_64-cmake
+
+# Install NSIS64 for Release packaging
+pacman -S mingw-w64-x86_64-nsis
+```
+
+## How to build
+
+On Linux and Darwin, create a `Debug` or `Release` directory.
+
+```sh
+# For debugging
+mkdir Debug
+cd Debug
+cmake -DCMAKE_BUILD_TYPE=Debug ..
+make -j
+
+# For releasing
+mkdir Release
+cd Release
+cmake -DCMAKE_BUILD_TYPE=Release ..
+make -j
+```
+
+In the case of Windows with [MSYS2][3], it's a bit different because you need to
+specify the right generator (`MSYS Makefiles`).
+
+```sh
+# For debugging
+mkdir Debug
+cd Debug
+cmake -G"MSYS Makefiles" -DCMAKE_BUILD_TYPE=Debug ..
+make -j
+
+# For releasing
+mkdir Release
+cd Release
+cmake -G"MSYS Makefiles" -DCMAKE_BUILD_TYPE=Release ..
+make -j
+```
+
+## Packaging
+
+If you have built a release, you will find the output in the appropriate
+directory according to the platform.
+
+-   **Linux**: look at `Release/linux-appimage`
+-   **Darwin**: look at `Release/src/planetblupi_Project/???`
+-   **Windows**: look at `Release/src/planetblupi_Project/planetblupi-X.Y.Z.exe`
+
+
+[1]: http://appimage.org
+[2]: http://nsis.sourceforge.net
+[3]: http://www.msys2.org
+[4]: https://gcc.gnu.org
+[5]: https://clang.llvm.org
+[6]: https://cmake.org