Update docs/Manifest.md to include "compatible"

Change-Id: I085e61a647814d5454f15d452ccab15ed4b50d1f
diff --git a/docs/Manifest.md b/docs/Manifest.md
index 226480f..00dd86f 100644
--- a/docs/Manifest.md
+++ b/docs/Manifest.md
@@ -10,6 +10,8 @@
 
 &{/} {
 	hypervisor {
+		compatible = "hafnium,hafnium";
+
 		vm1 {
 			debug_name = "name";
 			kernel_filename = "vmlinuz";
@@ -47,6 +49,8 @@
 
 &{/} {
 	hypervisor {
+		compatible = "hafnium,hafnium";
+
 		vm1 {
 			debug_name = "primary VM";
 			kernel_filename = "vmlinuz";