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).

README.md

@@ -101,6 +101,18 @@ There is a lot more documentation in the [wiki].
 
 [wiki]: https://github.com/rust-lang/rust/wiki
 
+## Getting help and getting involved
+
+The Rust community congregates in a few places:
+
+* [StackOverflow] - Get help here.
+* [/r/rust] - General discussion.
+* [discuss.rust-lang.org] - For development of the Rust language itself.
+
+[StackOverflow]: http://stackoverflow.com/questions/tagged/rust
+[/r/rust]: http://reddit.com/r/rust
+[discuss.rust-lang.org]: http://discuss.rust-lang.org/
+
 ## License
 
 Rust is primarily distributed under the terms of both the MIT license