[gce] Document create-instance/destroy-instance subcommands

fx gce create-instance/destroy-instance commands create or destroy
a gce instance based on the most recently uploaded fuchsia image.

Change-Id: I12dfaa76454f2522c354ba8e67be7359d5e6692e
diff --git a/gce/README.md b/gce/README.md
index 011e2cf..8b32522 100644
--- a/gce/README.md
+++ b/gce/README.md
@@ -31,6 +31,7 @@
 fx gce create-instance
 sleep 60
 fx gce serial
+fx gce delete-instance
 ```
 
 ## How the sausage is made
@@ -62,3 +63,6 @@
    have an appropriately configured compute engine ssh key for this to work. If
    you have a more tuned ssh configuration, you may need to add
    `~/.ssh/google_compute_engine` to your `ssh-agent`.
+ * create-instance - create a GCE instance running fuchsia based on the most
+   recently created fuchsia image.
+ * delete-instance - deletes a GCE instance created by `create-instance`.