Add links to forums in README.md, including discuss.rust-lang.org

This adds links to SO, reddit, and discuss to the README. The main intent is to start advertising discuss.rust-lang.org more, in a location that doesn't mislead casual users to it (people who are building Rust are more likely to be the right audience than those that are just visiting the web site).
this should be `[/r/rust]`, with the preceeding `/`