Add variables to README. [ci skip]

README.md

@@ -12,9 +12,22 @@ None.
 
 Available variables are listed below, along with default values (see `defaults/main.yml`):
 
-    TODO.
+    docker_package: "docker-engine"
 
-TODO.
+The Docker package to install.
+
+    docker_install_compose: true
+    docker_compose_version: "1.11.2"
+
+Docker Compose installation options.
+
+    docker_apt_repository: "deb https://apt.dockerproject.org/repo {{ ansible_distribution|lower }}-{{ ansible_distribution_release }} main"
+
+(Used only for Debian/Ubuntu.) Add 'testing' if you don't want stable.
+
+    docker_yum_repo_version: 'main' # 'testing', 'beta', 'nightly'
+
+(Used only for RedHat/CentOS.)
 
 ## Dependencies