Add an optional metadata section

[msuchane]

Optionally, add a comment block near the start of the generated file that would store certain metadata useful to writers or to Pantheon.

Internal metadata candidates:

• Supported versions: ex.: 7, 8, 9
• Last review (by QE): ex.: 7.9, 8.3
• Tracker ticket, ex. : RHELPLAN-14115, BZ#11265666
• SMEs
• User story definition

The exact format of the metadata is up to discussion. It might be useful for the section to be parsable, but it's unclear if the benefit outweighs the cost of maintaining the validity, educating writers, etc.