Added Rust official discord in RESOURCES.md
The rust programming language on discord is a great place to discuss about the language and for quick queries. Added the following links in the resources.
* [Stack Overflow](http://stackoverflow.com/questions/tagged/rust) can be used to search for your problem and see if it has been answered already. You can also ask and answer questions.
* The [Rust User Forum](http://users.rust-lang.org) is for general discussion about Rust.
* [/r/rust](http://www.reddit.com/r/rust/) is the official Rust subreddit.
+* [The Rust Programming Language](https://discordapp.com/invite/rust) official server on [Discord](https://discordapp.com/) can be used for quick queries and discussions about the language.
