chore(repo): added readme

src/repo/README.md

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+# repo
+
+This module manages the contents of the various repositories stored within a
+Vieter instance.
+
+## Terminology
+
+* Arch-repository (arch-repo): specific architecture of a given repository. This is what
+  Pacman actually uses as a repository, and contains its own `.db` & `.files`
+  files.
+* Repository (repo): a collection of arch-repositories. A single repository can
+  contain packages of different architectures, with each package being stored
+  in that specific architecture' arch-repository.
+* Repository group (repo-group): a collection of repositories. Each Vieter
+  instance consists of a single repository group, which manages all underlying
+  repositories & arch-repositories.
+
+## Arch-repository layout
+
+An arch-repository (aka a regular Pacman repository) consists of a directory
+with the following files (`{repo}` should be replaced with the name of the
+repository):
+
+* One or more package directories. These directories follow the naming scheme
+  `${pkgname}-${pkgver}-${pkgrel}`. Each of these directories contains two
+  files, `desc` & `files`. The `desc` file is a list of the package's metadata,
+  while `files` contains a list of all files that the package contains. The
+  latter is used when using `pacman -F`.
+* `{repo}.db` & `{repo}.db.tar.gz`: the database file of the repository. This
+  is just a compressed tarball of all package directories, but only their
+  `desc` files. Both these files should have the same content (`repo-add`
+  creates a symlink, but Vieter just serves the same file for both routes)
+* `{repo}.files` & `{repo}.files.tar.gz`: the same as the `.db` file, but this
+  also contains the `files` files, instead of just the `desc` files.
+
+## Filesystem layout
+
+The repository part of Vieter consists of two directories. One is the `repos`
+directory inside the configured `data_dir`, while the other is the configured
+`pkg_dir`. `repos` contains only the repository group, while `pkg_dir` contains
+the actual package archives. `pkg_dir` is the directory that can take up a
+significant amount of memory, while `repos` solely consists of small text
+files.