Improve chapter 1 with more linkage to Rust resources, and a full line-by-line explanation of the Hello Rust program. Also remove redundant extern crate, since Rust 2018 has obviated it.

.vscode/spellright.dict

@@ -22,3 +22,4 @@ bitset
 Bitsets
 Gedminas
 Kyzrati
+gotchas

chapter-01-hellorust/src/main.rs

@@ -1,4 +1,3 @@
-extern crate rltk;
 use rltk::{Rltk, GameState, Console};
 
 struct State {}