Update Conclusion of The Rust Programming Language

This replaces a obsolete conclusion (based on the original, basic Guide) with
links to further documentation sources.

src/doc/trpl/conclusion.md

@@ -1,10 +1,11 @@
 % Conclusion
 
 We covered a lot of ground here. When you've mastered everything in this Guide,
-you will have a firm grasp of basic Rust development. There's a whole lot more
-out there, we've just covered the surface. There's tons of topics that you can
-dig deeper into, and we've built specialized guides for many of them. To learn
-more, dig into the [full documentation
-index](index.html).
+you will have a firm grasp of Rust development. There's a whole lot more
+out there, though, we've just covered the surface. There's tons of topics that
+you can dig deeper into, e.g. by reading the API documentation of the
+[standard library](http://doc.rust-lang.org/std/), by discovering solutions for
+common problems on [Rust by Example](http://rustbyexample.com/), or by browsing
+crates written by the community on [crates.io](https://crates.io/).
 
 Happy hacking!