docs: image support marked as completed (#1008)

## Summary

* **What is the goal of this PR?** (e.g., Implements the new feature for
file uploading.)

Epub image support was added in #556. The goal of this PR is to document
that in the readme.

* **What changes are included?**

Only the checkmark in the readme.

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks,
specific areas to focus on).

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### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**< NO >**_