Example for saving assets

## What problem does this solve or what need does it fill?

All of the official asset-related examples are limited to reading, not writing, assets. I realize of course that this is a difficult issue because the examples are intended to run on the web, where writing anything is hard.

However, for someone whose goals are to build some kind of editor, and who wants to know what are the best practices and techniques for saving assets, there's essentially nothing in the official docs or examples that tells them where to start.

## What solution would you like?

Either an example, or if that is not possible, at least a mention in the documentation that points to some github repo that would give a starting point for writing assets.