Add 'assets' API endpoint to the docs

[owenvoke]

Description

Add the ?q=assets API endpoint to the docs, as per #12.

Types of changes

Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

Put an x in all the boxes that apply:

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.

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