Rollup merge of rust-lang#25321 - steveklabnik:second_tutorial, r=alexcrichton

This is a little rough, and it needs squashed and section headers, but i'd like to get some eyes on it sooner rather than later.

Author: steveklabnik

src/doc/trpl/SUMMARY.md

@@ -6,6 +6,7 @@
     * [Hello, Cargo!](hello-cargo.md)
 * [Learn Rust](learn-rust.md)
     * [Guessing Game](guessing-game.md)
+    * [Dining Philosophers](dining-philosophers.md)
 * [Effective Rust](effective-rust.md)
     * [The Stack and the Heap](the-stack-and-the-heap.md)
     * [Testing](testing.md)