Conventions: add "check mode" section

CONVENTIONS.md

@@ -182,6 +182,29 @@ copy:
 
 The source file or template shouldn't to be prefixed for ordering (eg. `z-` or `zzz-`). It's the task's responsibility to choose how destination files must be ordered.
 
+### check mode
+
+Ansible can run playbooks in "check mode" with the `--check` argument of `ansible-playbook`. It won't do any changing action but will report what would be changed or not.
+
+When an action depends on a variable registered by a previous action (that performs no change, like a grep, stat…) we should add a `always_run: yes` to the action. In Ansible 2.2, the behaviour has been replace by `check_mode: yes.` Exemple :
+
+```
+- name: Check if Munin plugins exists
+  stat:
+    path: /etc/munin/plugins/
+  register: munin_plugins_dir
+  #check_mode: no (for migration to Ansible 2.2)
+  always_run: yes
+
+- name: Get Munin plugin
+  copy:
+    src: munin/drbd-plugin
+    dest: /etc/munin/plugins/drbd
+    mode: "755"
+  when: munin_plugins_dir.stat.exists
+  notify: restart munin-node
+```
+
 ### packages installation
 
 When making a role or a task the necessary packages must be installed explicitly.