Add section in the FAQ on new image proposals

[wglambert]

Giving a brief overview to the question "Is $project a good fit to be included in the library?" from the suggestion over in docker-library/official-images#6282 (comment)

There is more detail on this at https://github.com/docker-library/official-images/blob/master/README.md so a quick FAQ entry of the high-points would give a concise and referenceable answer for any prospective contributors

[tianon]

This is a good start, thanks!

I'd like to move it up a bit higher (just below How are images built? (especially multiarch) and above What is bashbrew? Where can I download it?), and do a bit of wordsmithing (hopefully your goal was to force me to focus on this long enough to finally write something): 😇 😅

### Inclusion Criteria

See ["What are 'Official Images'?" in the main project repository](https://github.com/docker-library/official-images#what-are-official-images) for a high-level overview of the focus and goals of the project in general.

Per [the "New Image Checklist"](https://github.com/docker-library/official-images/blob/master/NEW-IMAGE-CHECKLIST.md) (which is used to roughly track status during "New Image" reviews), one of the primary determinations we try to make is whether the image being proposed is "generally useful" and whether the software itself is "reasonably popular" and/or "solves a particular use case well" (to help focus our review bandwidth on things that will be helpful to as large a set of users as possible).

There are also specific `Dockerfile` writing guidelines which can be found in [the "Review Guidelines" section of our documentation](https://github.com/docker-library/official-images#review-guidelines), which is used as a basis for a lot of our review process.

(I'm also going to explicitly cc @yosifkit for more review/thoughts and @samuelkarp who I'm hoping will be willing to offer some opinions on whether we're near the target.)

Also, since GitHub makes that code block so hard to look at, here it is rendered too:

Inclusion Criteria

See "What are 'Official Images'?" in the main project repository for a high-level overview of the focus and goals of the project in general.

Per the "New Image Checklist" (which is used to roughly track status during "New Image" reviews), one of the primary determinations we try to make is whether the image being proposed is "generally useful" and whether the software itself is "reasonably popular" and/or "solves a particular use case well" (to help focus our review bandwidth on things that will be helpful to as large a set of users as possible).

There are also specific Dockerfile writing guidelines which can be found in the "Review Guidelines" section of our documentation, which is used as a basis for a lot of our review process.