docs： add /explain-this-file custom command example to README。

[94tianqi]

This PR adds a complete example for a custom slash command /explain-this-file to the README's "Custom Slash Commands" section, as discussed in #106.

Why useful:

• Demonstrates how to use frontmatter + multi-step reasoning for a common task (explaining files).
• Complements existing guidance without examples in README.
• Helps new users get started with reusable prompts quickly.

Changes:

• Added a new subsection under "Custom Slash Commands" with file path .augment/commands/explain-this-file.md.
• Included frontmatter (description), full prompt template, and usage note.

Related issue: #106 (where justinux421 invited PR submission)

Happy to adjust wording, add more examples, or refine the prompt!

[justinxu421]

Could you add this to this folder: https://github.com/augmentcode/auggie/tree/main/examples/commands

[94tianqi]

@justinux421 Done! Added explain-this-file.md to /examples/commands as requested.
Also added a small reference in README for better discoverability.

Ready for review/merge. Thanks!