README: add runtime, bundle, and hook author user

Lets call out some users directly and give them titles. Then define what
they is trying to do.

Signed-off-by: Brandon Philips <brandon.philips@coreos.com>

README.md

@@ -22,10 +22,20 @@ In the specifications in the above table of contents, the keywords "MUST", "MUST
 
 To provide context for users the following section gives example use cases for each part of the spec.
 
-#### Filesystem Bundle & Configuration
+#### Application Bundle Builders
 
-- A user can create a root filesystem and configuration, with low-level OS and host specific details, and launch it as a container under an Open Container runtime.
+Application bundle builders can create a [bundle](bundle.md) directory that includes all of the files required for launching an application as a container.
+The bundle contains OCI [configuration files](config.md) where the builder can specify host-independent details such as [which executable to launch](config.md#process-configuration) and host-specific settings such as [mount](runtime-config.md#mount-configuration) locations, [hook](runtime-config.md#hooks) paths, Linux [namespaces](runtime-config-linux.md#namespaces) and [cgroups](runtime-config-linux.md#control-groups).
+Because the configuration includes host-specific settings, application bundle directories copied between two hosts may require configuration adjustments.
 
+#### Hook Developers
+
+[Hook](runtime-config.md#hooks) developers can extend the functionality of an OCI-compliant runtime by hooking into a container's lifecycle with an external application.
+Example use cases include sophisticated network configuration, volume garbage collection, etc.
+
+#### Runtime Developers
+
+Runtime developers can build runtime implementations that run OCI-compliant bundles and container configuration, containing low-level OS and host specific details, on a particular platform.
 
 # Releases