book: define trait

tshepang · tshepang · commit e2a1afa1c2f3 · 2015-06-09T02:32:29.000+02:00
diff --git a/src/doc/trpl/traits.md b/src/doc/trpl/traits.md
@@ -1,5 +1,8 @@
 % Traits
 
+A trait is a language feature that tells the Rust compiler about
+functionality a type must provide.
+
 Do you remember the `impl` keyword, used to call a function with [method
 syntax][methodsyntax]?
 
