add some known tweaks that need to be made for coreos

docs/coreos.md

@@ -11,6 +11,10 @@ Or with Ansible:
 
 Before running the cluster playbook you must satisfy the following requirements:
 
-* On each CoreOS nodes a writable directory **/opt/bin** (~400M disk space)
+General CoreOS Pre-Installation Notes:
+- You should set the bootstrap_os variable to `coreos`
+- Ensure that the bin_dir is set to `/opt/bin`
+- ansible_python_interpreter should be `/opt/bin/python`. This will be laid down by the bootstrap task.
+- The default resolvconf_mode setting of `docker_dns` **does not** work for CoreOS. This is because we do not edit the systemd service file for docker on CoreOS nodes. Instead, just use the `host_resolvconf` mode. It should work out of the box.
 
 Then you can proceed to [cluster deployment](#run-deployment)