Added readme for insertor component

[vk17-starlord]

PR Description

What?
Closes Issue #52062

This PR adds a README.md file for the inserter component in the Gutenberg project.

Why?
The inserter component was missing a README.md file, which is important for providing documentation and clarity on the component's purpose and usage.

How?

• Created a new README.md file for the inserter component.
• Followed the structure and content style of the alignment-control README as a reference.

Testing Instructions

1. Navigate to the inserter component directory.
2. Ensure the new README.md file is present and contains relevant information about the component.
3. Review the content to verify it aligns with Gutenberg documentation standards.

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Co-authored-by: vk17-starlord <[email protected]>
Co-authored-by: mburridge <[email protected]>

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