Document how to add a new / custom build target

[AverageMarcus]

Introduce new documentation that covers how someone could add their own Make target (provider + OS combo).

This documentation should serve two purposes:

1. As part of contributing documentation to help people adding new OS options to existing providers as part of pull requests
2. As a way of manually adding custom (or old) operating systems not officially supported by the image-builder project.

This would not only help new contributors in helping grow the project but it would also allow people to continue running end-of-life releases if required. (e.g. #1791)

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/help
/lifecycle frozen

[k8s-ci-robot]

@AverageMarcus:
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In response to this:

Introduce new documentation that covers how someone could add their own Make target (provider + OS combo).

This documentation should serve two purposes:

1. As part of contributing documentation to help people adding new OS options to existing providers as part of pull requests
2. As a way of manually adding custom (or old) operating systems not officially supported by the image-builder project.

This would not only help new contributors in helping grow the project but it would also allow people to continue running end-of-life releases if required. (e.g. #1791)

---

/help
/lifecycle frozen

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