add 'description' parameter.

PROPOSAL: 'description' parameter

I'd like to add a "description" parameter.
Ansible does nothing about on it. it just describing what is it.

I know YAML can have a comment and a name is good to know what this task
do. But a comment is not structured and a name can not be long because
handlers use it.

If ansible defines "description", a documentation tool such as
Sphinx can read it and produces a good document.

```
---
- hosts: all
  description: This playbook is an example for describe directive
  tasks:
    - name: create empty file
      file: path=/tmp/empty state=touch
      description: >
        This empty file is needed to later tasks,
        such as A, B, and C.
    - name: do some script
      script: /some/local/script.sh
      description:
        - a
        - b
        - c
    - script: /some/local/script.sh
      description:
        can: use map
        some: thing
        other: thing

- include: other.yml
  description:
    - include can have description also
```


I will send this proposal to the ansible-project mailing list.