add docker_use_dist_repo variable + default

to decide for install source repo
and add to README

README.md

@@ -1,4 +1,4 @@
-# Ansible Collection - `dede.common`
+# Ansible Collection - dede.common
 
 this ansible collection primarily aims to provide common roles that i've been building
 for either my home or the wululu web network and systems.
@@ -6,4 +6,17 @@ for either my home or the wululu web network and systems.
 by publishing it i hope it can be useful for anyone looking for inspiration
 or even roles ready-to-go.
 
+please mind that everything in this collection mainly focuses debian as a server operating system.
+thus i will not add features specific to other operating systems unless i need to.
+
 if you have any questions feel free to get in touch with me.
+
+## roles
+
+* **test**
+  a simple role to test ansible or just this collection
+* **docker**
+  a docker install role that can install docker-ce engine in two ways:  
+  1. from the official docker repository (recommended by docker)
+  2. from your distribution
+  you can switch to the distribution method by setting `docker_use_dist_repo` to `true` (defaults to `false`).