message: Add readme sections about repository types
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  +[![Docs](https://img.shields.io/badge/docs-passing-brightgreen)](https://esrlabs.github.io/northstar/northstar/index.html)
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  @@ -207,15 +208,43 @@ type = "mem"
  # NPK Repository `default` in `dir`
  [repositories.default]
  # Mount the containers from this repository on runtime start. Default: false
  -mount = true
  +mount_on_start = true
  key = "examples/northstar.pub"
  type = { fs = { dir = "target/northstar/repository" }}
  ```
  ### Repositories
  -**TODO**: Describe what a repository is and what are the attributes:
  -*with/without key/verity, etc...
  +A repository is an entity that is able to store NPK's at runtime. Repositories
  +are configured at initialization time of the runtime. The repository
  +configuration cannot be changed at runtime. Each configured repository has a
  +unique identifier
  +[id](https://esrlabs.github.io/northstar/northstar/api/model/type.RepositoryId.html).
  +Currently two types of repositories exists: `fs` and `mem`.
  +
  +The `fs` type repositories are backed by file system storage. The configured
  +directory (`dir`) is used to store NPK's. If this directory is read only, no
  +additional install requests can be performed at runtime. If a `fs` repository
  +configuration contains a `key` field, the repository is treated as "verified".
  +The configured key is used to verify the signature of the containers manifest
  +and it's verity root hash. When the container is mounted, the verity root hash
  +is used to configure a device mapper verity devices that is mounted instead of
  +the contained Squashfs image.
  +
  +Repositories without a `key` are treated as trustful sources. No signature
  +checks are performed. The root filesystems are mounted *without* verity. A
  +possibly present verity root hash with in the NPK is ignored. Trusted
  +repositories are should on verified and read only file systems.
  +
  +Set the `mount_on_start` flag of a `fs` repository to `true` to make the runtime
  +mount *all* containers present at startup. The mount operations are done in
  +parallel.
  +
  +The `mem` repositories uses
  +[memfd](https://man7.org/linux/man-pages/man2/memfd_create.2.html) for it's
  +storage. No data is persistently stored during an installation of a container.
  +Obviously it's not possible to have NPK's preinstalled in a `mem` repository at
  +runtime startup. The `mem` repositories are mainly used for testing.
  ## Console
  