Add speaker notes for Iterator, and an example using FromIterator.

src/SUMMARY.md

@@ -129,6 +129,7 @@
   - [Default Methods](traits/default-methods.md)
   - [Important Traits](traits/important-traits.md)
     - [`Iterator`](traits/iterator.md)
+    - [`FromIterator`](traits/from-iterator.md)
     - [`From` and `Into`](traits/from-into.md)
     - [`Read` and `Write`](traits/read-write.md)
     - [`Add`, `Mul`, ...](traits/operators.md)

src/traits/from-iterator.md

@@ -0,0 +1,23 @@
+# FromIterator
+
+`FromIterator` lets you build a collection from an `Iterator`.
+
+```rust,editable
+fn main() {
+    let primes = vec![2, 3, 5, 7];
+    let prime_squares = primes.into_iter().map(|prime| prime * prime).collect::<Vec<_>>();
+}
+```
+
+<details>
+
+`Iterator` implements
+`fn collect<B>(self) -> B
+where
+    B: FromIterator<Self::Item>,
+    Self: Sized`
+
+There are also implementations which let you do cool things like convert an
+`Iterator<Item = Result<V, E>>` into a `Result<Vec<V>, E>`.
+
+</details>

src/traits/iterator.md

@@ -26,3 +26,10 @@ fn main() {
     }
 }
 ```
+
+<details>
+
+`IntoIterator` is the trait that makes for loops work. It is implemented by collection types such as
+`Vec<T>` and references to them such as `&Vec<T>` and `&[T]`. Ranges also implement it.
+
+</details>