README: add runtime, bundle, and hook author user

Brandon Philips · Brandon Philips · commit 1ca1981ce190 · 2015-12-25T07:50:46.000-08:00
Lets call out some users directly and give them titles. Then define what
they is trying to do.

Signed-off-by: Brandon Philips &lt;brandon.philips@coreos.com&gt;
diff --git a/README.md b/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 and some low-level details for launching an application as a container.
+The application can in-turn rely on certain OS configuration being available when it is launched inside of a container by a runtime.
+The OS configuration will include things such as environment variables, a working directory, mount points, and operating system permissions.
 
+#### Hook Writers
+
+[Hook](runtime-config.md#hooks) authors 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 Writer
+
+Runtime authors 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
 
