Document Rust dependencies & build steps

[ford-at-aws]

NOTE ⚠️

Your language's existing top-level README may already meet the acceptance criteria defined below. If this is the case, let me (@ford-at-aws) know and I'll review it and close your story if true.

User story

As an example code customer, I want to know which dependencies are required in order to build and execute the examples themselves, and how to resolve those dependencies myself after cloning the code.

So that...

I can use this code without having to guess. (and also so that the internal AWS team can create build/test automation).

Acceptance criteria

I know this is done when:

• Steps are documented in language-level README
• Includes your OS (or OS's where code has been verified to work)
• Includes all external dependencies or third-party libraries (and where to see them in the code, e.g. pom.xml)
• Includes steps to resolve dependencies or build source code that a user could follow
• All of the above is documented in the language-level README

[DavidSouther]

All rust code can be built at the top level with cargo check --all-targets, which is the standard rust build mechanism. Cargo can be installed with rustup, again, the standard rust tool. This works for all platforms supported by Rust.