Add a section describing how to install the role

This resolves #167.

README.md

@@ -30,6 +30,28 @@ None.
 
 None.
 
+## Installation ##
+
+This role can be installed via the command
+
+```bash
+ansible-galaxy ansible-galaxy install --role-file path/to/requirements.yml
+```
+
+where `requirements.yml` looks like
+
+```yaml
+---
+- name: skeleton
+  src: https://github.com/cisagov/skeleton-ansible-role
+```
+
+and may contain multiple roles.
+
+For more information about installing Ansible roles via a YAML file,
+please see [the `ansible-galaxy`
+documentation](https://docs.ansible.com/ansible/latest/galaxy/user_guide.html#installing-multiple-roles-from-a-file).
+
 ## Example Playbook ##
 
 Here's how to use it in a playbook: