tree: add basic HACKING.md with section for MCD

HACKING.md

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+⚠⚠⚠ THIS IS A LIVING DOCUMENT AND LIKELY TO CHANGE QUICKLY ⚠⚠⚠
+
+# Hacking on the MCD
+
+1. Create a cluster (e.g. using [libvirt](https://github.com/openshift/installer/blob/master/Documentation/dev/libvirt-howto.md))
+2. Delete the old node agent daemonset:
+
+    ```sh
+    oc delete ds tectonic-node-agent --namespace tectonic-system
+    ```
+
+    This will be fixed in the installer eventually.
+
+3. Run:
+
+    ```sh
+    sed s/node-configuration.v1.coreos.com/machineconfiguration.openshift.io/g /opt/tectonic/node-annotations.json >/etc/machine-config-daemon/node-annotations.json
+    ```
+
+    The install process will take care of this eventually.
+
+4. Create a namespace, e.g. `oc new-project mco`
+5. Create items from `manifests/` in a sensible order, e.g.
+
+    ```sh
+    for f in manifests/*.crd.yaml; do oc create -f $f; done
+    for f in manifests/*.machineconfigpool.yaml; do oc create -f $f; done
+    oc create -f manifests/machineconfigdaemon/clusterrole.yaml
+    oc create -f manifests/machineconfigdaemon/sa.yaml
+    oc create -f manifests/machineconfigdaemon/scc.yaml
+    oc create -f manifests/machineconfigdaemon/clusterrolebinding.yaml
+    oc create -f manifests/machineconfigdaemon/daemonset.yaml
+    ```
+
+    (Though note you'll have to sub in `{{.TargetNamespace}}` with `mco`).