Improve README with detailed project overview and usage examples

[tongshan77]

This PR improves the Reindeer project's README by:

• Adding a detailed project overview explaining Reindeer's purpose and core features.
• Providing usage examples to demonstrate how to import Rust crates and generate Buck build rules.
• Geting Help information

[facebook-github-bot]

Hi @tongshan77!

Thank you for your pull request and welcome to our community.

Action Required

In order to merge any pull request (code, docs, etc.), we require contributors to sign our Contributor License Agreement, and we don't seem to have one on file for you.

Process

In order for us to review and merge your suggested changes, please sign at https://code.facebook.com/cla. If you are contributing on behalf of someone else (eg your employer), the individual CLA may not be sufficient and your employer may need to sign the corporate CLA.

Once the CLA is signed, our tooling will perform checks and validations. Afterwards, the pull request will be tagged with CLA signed. The tagging process may take up to 1 hour after signing. Please give it that time before contacting us about it.

If you have received this in error or have any questions, please contact us at cla@meta.com. Thanks!