feat(coreos.oem): Write coreos.oem fields to /etc/oem-release

Documentation/cloud-config.md

@@ -39,6 +39,45 @@ Note that hyphens in the coreos.etcd.* keys are mapped to underscores.
 
 [etcd-config]: https://github.com/coreos/etcd/blob/master/Documentation/configuration.md
 
+### coreos.oem
+
+These fields are borrowed from the [os-release spec][os-release] and repurposed
+as a way for coreos-cloudinit to know about the OEM partition on this machine:
+
+- **id**: Lowercase string identifying the OEM
+- **name**: Human-friendly string representing the OEM
+- **version-id**: Lowercase string identifying the version of the OEM
+- **home-url**: Link to the homepage of the provider or OEM
+- **bug-report-url**: Link to a place to file bug reports about this OEM
+
+coreos-cloudinit renders these fields to `/etc/oem-release`.
+If no **id** field is provided, coreos-cloudinit will ignore this section.
+
+For example, the following cloud-config document...
+
+```
+#cloud-config
+coreos:
+  oem:
+    id: rackspace
+    name: Rackspace Cloud Servers
+    version-id: 168.0.0
+    home-url: https://www.rackspace.com/cloud/servers/
+    bug-report-url: https://github.com/coreos/coreos-overlay
+```
+
+...would be rendered to the following `/etc/oem-release`:
+
+```
+ID="rackspace"
+NAME="Rackspace Cloud Servers"
+VERSION_ID="168.0.0"
+HOME_URL="https://www.rackspace.com/cloud/servers/"
+BUG_REPORT_URL="https://github.com/coreos/coreos-overlay"
+```
+
+[os-release]: http://www.freedesktop.org/software/systemd/man/os-release.html
+
 ### coreos.units
 
 Arbitrary systemd units may be provided in the `coreos.units` attribute.