Rust unstable book: basic desc and example for const_fn.

src/doc/unstable-book/src/const-fn.md

@@ -6,5 +6,24 @@ The tracking issue for this feature is: [#24111]
 
 ------------------------
 
+The `const_fn` feature allows marking free functions and inherent methods as
+`const`, enabling them to be called in constants contexts, with constant
+arguments.
 
+## Examples
 
+```rust
+#![feature(const_fn)]
+
+const fn double(x: i32) -> i32 {
+    x * 2
+}
+
+const FIVE: i32 = 5;
+const TEN: i32 = double(FIVE);
+
+fn main() {
+    assert_eq!(5, FIVE);
+    assert_eq!(10, TEN);
+}
+```