Add links to forums in README.md, including discuss.rust-lang.org
auto merge of #16680 : brson/rust/readme, r=steveklabnik

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).