Rust: make sure there are prominent instructions about where to ask for help

[brson]

I much want course takers to get a taste of the Rust community while learning how to learn Rust. The venues at which to ask for help should be emphasized at the beginning of the documentation, and reiterated elsewhere.

#beginners discord, #rust-beginners irc, Rust China Community WeChat

users.rust-lang.org, stack overflow

[brson]

It would also be good to have a place to discuss the course itself.