docs: Update platform docs

- Introduce a table of supported platforms in top-level README
- Add stub files for un-documented platforms using a standard template
- Update HyperKit doc to follow a standard template

Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>

docs/platform-qemu.md

@@ -1,7 +1,33 @@
-# LinuxKit and qemu
+# LinuxKit with qemu/kvm
 
 TBD
 
+## Boot
+
+TBD: Describe the different boot modes (kernel+initrd, iso, efi, disk etc)
+
+
+## Console
+
+TBD: 
+
+
+## Disks
+
+The qemu backend supports multiple disks to be attached to the VM
+using the standard `linuxkit` `-disk` syntax. The qemu backend
+supports a number of different disk formats.
+
+
 ## Networking
 
 TBD
+
+Also explain port forwarding
+
+## Integration services and Metadata
+
+TBD: Integration services
+
+The qemu backend currently does not support passing metadata/userdata
+to the VM.