Added contrib/ directory with Doxyfile and ArchLinux PKGBUILD file

Updated the README.md with short build instructions and mentioning
doxygen as optional dependency.
The new contrib/ directory will host platform-specific stuff for
installation purposes. Documentation can now be generated using the
included Doxyfile, or by running "make docs" in the build directory.
Also wrote a PKGBUILD file for ArchLinux which builds the currently
latest git commit of this repository into a installable package.