Add Cargo.toml example to README

[shinglyu]

Issue #, if available: N/A

Description of changes:
Added a Cargo.toml example for the example code in README. This helps new users get started with the example easier.

By submitting this pull request

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.

[shinglyu]

I'm using "*" to avoid having to update this README everytime a new version is released.

[calavera]

I would prefer to document how to get started using cargo and cargo-lambda than maintaining this static code. Telling people to use '*' as dependency is not a great general practice, and people won't think twice to copy it.

Generating a valid project is as easy as running cargo lambda new, see this documentation. Or, if people don't want to use cargo-lambda, we can also say "run cargo new, copy the code inside, and add the lambda_runtime dependency to the generated Cargo.toml".

[calavera]

Thanks for opening helping us improve our docs!

I've added new information to get started in #484. I'm going to close this PR for the time being.