docs: Add a Usage section

Add a new Usage section with basic examples of how to run the
builders from the `Makefile`.

Fixes #84.

Signed-off-by: James O. D. Hunt <james.o.hunt@intel.com>

README.md

@@ -4,6 +4,14 @@
 
 * [Introduction](#introduction)
 * [Terms](#terms)
+* [Usage](#usage)
+    * [Rootfs creation](#rootfs-creation)
+        * [Rootfs with systemd as init](#rootfs-with-systemd-as-init)
+        * [Rootfs with the agent as init](#rootfs-with-the-agent-as-init)
+    * [Image creation](#image-creation)
+        * [Image with systemd as init](#image-with-systemd-as-init)
+        * [Image with the agent as init](#image-with-the-agent-as-init)
+    * [Initrd creation](#initrd-creation)
 
 ## Introduction
 
@@ -20,10 +28,9 @@ This section describes the terms used for all documentation in this repository.
 
 - rootfs
 
-  The root filesystem or "rootfs" is the set of files contained in the
+  The root filesystem or "rootfs" is a slight misnomer as it is not a true filesystem. It is a tree of files contained in a particular directory, which represents the root disk layout. A rootfs can be turned into either an image or an initrd.
-  guest root directory that builds into a filesystem.
 
-  See [the rootfs builder documentation](rootfs-builder/README.md).
+  See the [rootfs creation](#rootfs-creation) section.
 
 - "Guest OS" (or "Guest Image")
 
@@ -32,16 +39,71 @@ This section describes the terms used for all documentation in this repository.
   create an environment to host the container. Neither the guest OS nor the
   guest kernel need to be the same as the host operating system.
 
-  See [the image builder documentation](image-builder/README.md).
+  See the [image creation](#image-creation) section.
 
 - initrd (or "initramfs")
 
-  A compressed cpio archive loaded into memory and used as part of the Linux
+  A compressed `cpio(1)` archive, created from a rootfs which is loaded into memory and used as part of the Linux startup process. During startup, the kernel unpacks it into a special instance of a `tmpfs` that becomes the initial root filesystem.
-  startup process. During startup, the kernel unpacks it into a special
-  instance of a tmpfs that becomes the initial root file system.
 
-  See [the initrd builder documentation](initrd-builder/README.md).
+  See the [initrd creation](#initrd-creation) section.
 
 - "Base OS"
 
-  A particular version of a Linux distribution used to create a Guest OS from.
+  A particular version of a Linux distribution used to create a rootfs from.
+
+## Usage
+
+The top-level `Makefile` contains an example of how to use the available components.
+
+By default, components will run on the host system. However, some components
+offer the ability to run from within Docker (for ease of setup) by setting the
+`USE_DOCKER=true` variable.
+
+For more detailed information, consult the documentation for a particular component.
+
+### Rootfs creation
+
+This section shows how to build a basic rootfs using the default distribution.
+For further details, see
+[the rootfs builder documentation](rootfs-builder/README.md).
+
+#### Rootfs with systemd as init
+
+```
+$ sudo -E PATH=$PATH make USE_DOCKER=true rootfs
+```
+
+#### Rootfs with the agent as init
+
+```
+$ sudo -E PATH=$PATH make USE_DOCKER=true AGENT_INIT=yes rootfs
+```
+
+### Image creation
+
+This section shows how to create an image from the already-created rootfs. For
+further details, see
+[the image builder documentation](image-builder/README.md).
+
+#### Image with systemd as init
+
+```
+$ sudo -E PATH=$PATH make USE_DOCKER=true image-only
+```
+
+#### Image with the agent as init
+
+```
+$ sudo -E PATH=$PATH make USE_DOCKER=true AGENT_INIT=yes image-only
+```
+
+### Initrd creation
+
+To create an initrd from the already-created rootfs with the agent acting as the init daemon:
+
+```
+$ sudo -E PATH=$PATH make AGENT_INIT=yes initrd-only
+```
+
+For further details,
+see[the initrd builder documentation](initrd-builder/README.md).