Add support for automation description

[emontnemery]

Description:

When using the automation editor, any manually added comments are wiped.
Allow adding a description string to the automation instead, with the same purpose as a comment.

Pull request with documentation for home-assistant.io (if applicable): home-assistant/home-assistant.io#

Example entry for configuration.yaml (if applicable):

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the [development checklist][dev-checklist]

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated in home-assistant.io

If the code does not interact with devices:

• Tests have been added to verify that the new code works.

[emontnemery]

Are tests needed for this?

[MartinHjelmare]

Probably, but wait for a core review if this feature hasn't already been discussed.

[pvizeli]

I mean there is an proposal on architecture repository. I would suggest to implement this proposal.

[emontnemery]

@pvizeli I can't find it, can you share the issue number?

[pvizeli]

home-assistant/architecture#275