HOOKS.md added documenting new hooks

HOOKS.md

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+# Hooks
+
+The ansible installer allows for operators to execute custom tasks during
+specific operations through a system called hooks. Hooks allow operators to
+provide files defining tasks to execute before and/or after specific areas
+during installations and upgrades. This can be very helpful to validate
+or modify custom infrastructure when installing/upgrading OpenShift.
+
+It is important to remember that when a hook fails the operation fails. This
+means a good hook can run multiple times and provide the same results. A great
+hook is idempotent.
+
+**Note**: There is currently not a standard interface for hooks. In the future
+a standard interface will be defined and any hooks that existed previously will
+need to be updated to meet the new standard.
+
+## Using Hooks
+
+Hooks are defined in the ``hosts`` inventory file under the ``OSEv3:vars``
+section.
+
+Each hook should point to a yaml file which defines Ansible tasks. This file
+will be used as an include meaning that the file can not be a playbook but
+a set of tasks. Best practice suggests using absolute paths to the hook file to avoid any ambiguity.
+
+### Example
+```ini
+[OSEv3:vars]
+# <snip>
+openshift_master_upgrade_pre_hook=/usr/share/custom/pre_master.yml
+openshift_master_upgrade_hook=/usr/share/custom/master.yml
+openshift_master_upgrade_post_hook=/usr/share/custom/post_master.yml
+# <snip>
+```
+
+Hook files must be a yaml formatted file that defines a set of Ansible tasks.
+The file may **not** be a playbook.
+
+### Example
+```yaml
+---
+# Trivial example forcing an operator to ack the start of an upgrade
+# file=/usr/share/custom/pre_master.yml
+
+- name: note the start of a master upgrade
+  debug:
+      msg: "Master upgrade of {{ inventory_hostname }} is about to start"
+
+- name: require an operator agree to start an upgrade
+  pause:
+      prompt: "Hit enter to start the master upgrade"
+```
+
+## Upgrade Hooks
+
+### openshift_master_upgrade_pre_hook
+- Runs **before** each master is upgraded.
+- This hook runs against **each master** in serial.
+- If a task needs to run against a different host, said task will need to use [``delegate_to`` or ``local_action``](http://docs.ansible.com/ansible/playbooks_delegation.html#delegation).
+
+### openshift_master_upgrade_hook
+- Runs **after** each master is upgraded but **before** it's service/system restart.
+- This hook runs against **each master** in serial.
+- If a task needs to run against a different host, said task will need to use [``delegate_to`` or ``local_action``](http://docs.ansible.com/ansible/playbooks_delegation.html#delegation).
+
+
+### openshift_master_upgrade_post_hook
+- Runs **after** each master is upgraded and has had it's service/system restart.
+- This hook runs against **each master** in serial.
+- If a task needs to run against a different host, said task will need to use [``delegate_to`` or ``local_action``](http://docs.ansible.com/ansible/playbooks_delegation.html#delegation).