Add a slide about writing unsafe functions.

src/unsafe/calling-unsafe-functions.md

@@ -0,0 +1,18 @@
+# Calling Unsafe Functions
+
+A function or method can be marked `unsafe` if it has extra preconditions you
+must uphold to avoid undefined behaviour:
+
+```rust,editable
+fn main() {
+    let emojis = "🗻∈🌏";
+
+    // Safe because the indices are in the correct order, within the bounds of
+    // the string slice, and lie on UTF-8 sequence boundaries.
+    unsafe {
+        println!("{}", emojis.get_unchecked(0..4));
+        println!("{}", emojis.get_unchecked(4..7));
+        println!("{}", emojis.get_unchecked(7..11));
+    }
+}
+```