bundle.md: various updates to latest spec

- Move to single root filesystem, as the configuration specification now
  only supports a single process
- Clarify that signatures are just another kind of content directory
- Cross-reference configuration specification for config.json

Author: jonboulle

bundle.md

@@ -12,20 +12,19 @@ A standard container bundle is made of the following 3 parts:
 
 # Directory layout
 
-A Standard Container bundle is a directory containing all the content needed to load and run a container. This includes its configuration file, content directories, and cryptographic signatures. The main property of this directory layout is that it can be moved as a unit to another machine and run the same container.
+A Standard Container bundle is a directory containing all the content needed to load and run a container. This includes its configuration file (`config.json`) and content directories. The main property of this directory layout is that it can be moved as a unit to another machine and run the same container.
 
-One or more *content directories* may be adjacent to the configuration file. This at least includes the root filesystem (referenced in the configuration by the *rootfs* field) and other related content (signatures, other configs, etc.). The interpretation of these resources is specified in the configuration.
+The syntax and semantics for `config.json` are described in [this specification](config.md).
+
+One or more *content directories* may be adjacent to the configuration file. This must include at least the root filesystem (referenced in the configuration file by the *root* field) and may include other related content (signatures, other configs, etc.). The interpretation of these resources is specified in the configuration.
 
 ```
 /
 !
 -- config.json
 !
---- rootfs1
+--- rootfs
 !
---- rootfs2
+--- sigs
 ```
 
-The syntax and semantics for config.json are described in this specification.
-
-One or more content directories can be specified as root file systems for containers. They COULD be called rootfs..10^100 but SHALL be called whatever you want.