Add README entry about the new st2_packs variable

Author: armab

README.md

@@ -35,6 +35,7 @@ Below is the list of variables you can redefine in your playbook to customize st
 | **st2**
 | `st2_version`            | `latest`      | StackStorm version to install. Use latest `latest` to get automatic updates or pin it to numeric version like `2.1.1`.
 | `st2_revision`           | `1`           | StackStorm revision to install. Used only with pinned `st2_version`.
+| `st2_packs`              | `[]`          | A list of integration packs to install from [StackStorm Exchange](https://exchange.stackstorm.org/) or GitHub.
 | `st2_system_user`        | `stanley`     | System user from which st2 will execute local/remote shell actions.
 | `st2_system_user_in_sudoers` | `yes`| Add `st2_system_user` to the sudoers (recommended for most `st2` features to work).
 | `st2_ssh_key_file`       | `/home/{{st2_system_user}}/.ssh/{{st2_system_user}}_rsa` | Path to `st2_system_user` SSH private key. It will be autogenerated by default.

roles/st2/defaults/main.yml

@@ -6,6 +6,9 @@ st2_version: latest
 # used only if 'st2_version' is numeric
 st2_revision: 1
 
+# List of integration packs to install from StackStorm Exchange or GitHub
+st2_packs: []
+
 # System user from which st2 will execute local/remote shell actions
 st2_system_user: stanley
 # Add `st2_system_user` to the sudoers (recommended for most `st2` features to work)