Update vec.md (#240)

Adding Speaker Notes about type inference in collections and `vec![]` macro.

src/std/vec.md

@@ -4,9 +4,6 @@
 
 ```rust,editable
 fn main() {
-    let mut numbers = Vec::new();
-    numbers.push(42);
-
     let mut v1 = Vec::new();
     v1.push(42);
     println!("v1: len = {}, capacity = {}", v1.len(), v1.capacity());
@@ -15,6 +12,9 @@ fn main() {
     v2.extend(v1.iter());
     v2.push(9999);
     println!("v2: len = {}, capacity = {}", v2.len(), v2.capacity());
+    
+    let mut numbers = vec![1, 2, 3];
+    numbers.push(42);
 }
 ```
 
@@ -23,3 +23,13 @@ methods on a `Vec`.
 
 [1]: https://doc.rust-lang.org/std/vec/struct.Vec.html
 [2]: https://doc.rust-lang.org/std/vec/struct.Vec.html#deref-methods-[T]
+
+<details>
+    
+Notice how `Vec<T>` is a generic type too, but you don't have to specify `T` explicitly.
+As always with Rust type inference, the `T` was established during the first `push` call.
+
+`vec![...]` is a canonical macro to use instead of `Vec::new()` and it supports
+adding initial elements to the vector.
+
+</details>