Backport #640 to 1.2: Flesh out the csv a little

[webmat]

Backport of PR #640 to 1.2 branch. Original message:

In this PR:

• Add the field set column
• Add the short description
• Add whether the field should be indexed
• Reorder the columns a bit, so the stable length values are first

Preview GitHub's rendering of generated/csv/fields.csv (scroll right for definitions)

[webmat]

@ruflin Unsure what to do about messaging this backport. I wouldn't rev the ECS version, as the schema hasn't changed, only the csv file has.

Also, in 1.3 we're introducing the separation in the changelog for schema changes vs tooling/artifact changes. For 1.2.0 and before, we rarely messaged anything about tooling/artifact changes, so the changelog entries no longer have a home in older versions. I see two paths forward:

1. Just backport without any changelog entry, consistent with earlier versions' use of the changelog.
2. Retroactively introduce the changelog separation for all past releases, and reintroduce the changelog entries there.

I would tend to favor option 1, for simplicity's sake. WDYT?

[ruflin]

Why would you backport this in the first place?

[webmat]

To have the same format available for all versions.

Do you see a problem with this?