design: add azure section to cloud agents

Adds design decisions from coreos#65 & coreos#97.

Design.md

@@ -166,6 +166,13 @@ fields in the `/etc/os-release` file.
 
 Originally discussed in [#12](https://github.com/coreos/fedora-coreos-tracker/issues/12).
 
+### Azure:
+
+- We've identified one major gap with not shipping the [Microsoft Azure Linux Agent](https://github.com/Azure/WALinuxAgent): the machine will not check-in and will eventually be culled by Azure for being stuck in the creation process.
+- This gap will be temporarily worked around via a [systemd unit](https://github.com/coreos/fedora-coreos-tracker/issues/65#issuecomment-434449560) which will perform the check-in.
+- Long term this gap will be covered by work done in coreos/coreos-metadata#120
+- One additional gap which will not be covered is a lack of ephemeral disk support. We plan to ship udev rules but will not have a service which formats the disk unless we receive feature requests in the future.
+
 ### Summary:
 
  - FCOS will not ship cloud agents whenever possible.