[Rust] flatc instegration into build scripts (build.rs)

[frol]

I have released flatc-rust crate, which provides minimal programmatic interfaces to flatc CLI, which is useful for Cargo build scripts integration, thus making the development workflow consistent (i.e., you just use cargo build and the .fbs files are compiled to .rs transparently - example project).

Can I put some notes about flatc-rust into FlatBuffers documentation / tutorials?

[aardappel]

Ahh nice, that sounds very useful.

We typically don't explain external tools in the documentation itself, but adding a link to it would be good. Here https://google.github.io/flatbuffers/ we have a Useful documentation created by others, so maybe there could be a Useful tools created by others :)
Or maybe such a section specifically in the RustUsage.md.

[frol]

@aardappel PRed #5127