docs(alpm-mtree): MTree Specification

Fixes #38

So, this spec was a lot easier than expected.

The spec of the actual file is already well defined in its own mtree man page. The ALPM-MTREE spec now only describes the deviations to the actual mtree spec and how/where it's used.

Regarding how it's used:

As far as I've seen, Pacman uses mtree files only for validation of installed files via the -Qk or -Qkk flags. These commands to a comparison of the package's original file structure and metadata vs. the actual installed filestructure/metadata on the system. To prevent having to extract the whole package to get all files and their metadata, the mtree file has been added at the very start (hence the . to have it sorted at the very beginning :D) 0.

If no mtree file is found, the slow approach will be taken and all files will be inspected.