Add speaker notes for string.md

src/std/string.md

@@ -12,6 +12,10 @@ fn main() {
     s2.push_str(&s1);
     s2.push('!');
     println!("s2: len = {}, capacity = {}", s2.len(), s2.capacity());
+
+    let s3 = String::from("🇨🇭");
+    println!("s3: len = {}, number of chars = {}", s3.len(),
+             s3.chars().collect::<std::vec::Vec<_>>().len());
 }
 ```
 
@@ -20,3 +24,11 @@ fn main() {
 
 [1]: https://doc.rust-lang.org/std/string/struct.String.html
 [2]: https://doc.rust-lang.org/std/string/struct.String.html#deref-methods-str
+
+<details>
+
+* `len` returns the size of the `String` in bytes, not its length in characters.
+* `chars` returns an iterator over the actual characters.
+* `String` implements `Deref<Target = str>` which transparently gives it access to `str`'s methods.
+
+</details>