[gdk-pixbuf] docs: Fill out the README file

[Emmanuele Bassi <ebassi src gnome org>]

commit 5cbed24fc4f28dc2f158388e4e073ac8a0b1614f
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Thu Apr 26 14:27:32 2018 +0100

    docs: Fill out the README file
    
    It's desolate, at the moment, and free software projects without a
    decent README are a bit sad.

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+GdkPixbuf: Image loading library
+================================
+
+GdkPixbuf is a library that loads image data in various formats and stores
+it as linear buffers in memory. The buffers can then be scaled, composited,
+modified, saved, or rendered.
+
+GdkPixbuf can load image data encoded in different formats, such as:
+
+ - PNG
+ - JPEG
+ - TIFF
+ - TGA
+ - GIF
+
+Additionally, you can write a GdkPixbuf loader module and install it into
+a well-known location, in order to load a file format.
+
+GdkPixbuf is used by the [GTK](https://www.gtk.org) toolkit for loading
+graphical assets.
+
+## Building GdkPixbuf
+
+### Requirements
+
+In order to build GdkPixbuf you will need to have installed:
+
+ - Meson
+ - A C99-compliant compiler and toolchain
+ - GLib's development files
+
+Depending on the image formats you want to support you will also need:
+
+ - libpng's development files
+ - libjpeg's development files
+ - libtiff's development files
+ - libjasper's development files
+
+Additionally, you may need:
+
+ - shared-mime-info
+ - GObject Introspection
+ - GTK-Doc
+ - Xlib's development files
+ - mediaLib's development files
+
+### Building and installing
+
+You should use Meson to configure GdkPixbuf's build, and depending on the
+platform you will be able to use Ninja, Visual Studio, or XCode to build
+the project; typically, on most platforms, you should be able to use the
+following commands to build and install GdkPixbuf in the default prefix:
+
+```sh
+$ meson _build .
+$ cd _build
+$ ninja
+# ninja install
+```
+
+You can use the `--prefix` argument to control the installation prefix.
+
+You can also use `meson configure` from within the build directory to
+check the current build configuration, and change its options.
+
+## License
+
+GdkPixbuf is released under the terms of the GNU Library General Public
+License version 2, or, at your option, any later version. See the
+[COPYING](./COPYING) file for further details.