Add Speaker notes string.md (#368)

* Update string.md

* Update src/std/string.md

Co-authored-by: Martin Geisler <mgeisler@google.com>

* Update src/std/string.md

Co-authored-by: Martin Geisler <mgeisler@google.com>

* Update string.md

---------

Co-authored-by: Martin Geisler <mgeisler@google.com>

src/std/string.md

@@ -27,7 +27,14 @@ fn main() {
 
 <details>
 
-* `len()` returns the size of the `String` in bytes, not its length in characters.
-* `chars()` returns an iterator over the actual characters.
+* `String::new` returns a new empty string, use `String::with capacity` when you know how much data you want to push to the string.
+* `String::len` returns the size of the `String` in bytes (which can be different from its length in characters).
+* `String::chars` returns an iterator over the actual characters. Note that a `char` can be different from what a human will consider a "character" due to [grapheme clusters](https://docs.rs/unicode-segmentation/latest/unicode_segmentation/struct.Graphemes.html).
+*  When people refer to strings they could either be talking about `&str` or `String`.  
+* When a type implements `Deref<Target = T>`, the compiler will let you transparently call methods from `T`.
+    * `String` implements `Deref<Target = str>` which transparently gives it access to `str`'s methods.
+    * Write and compare `let s3 = s1.deref();` and  `let s3 = &*s1`;.
+* `String` is implemented as a wrapper around a vector of bytes, many of the operations you see supported on vectors are also supported on `String`, but with some extra guarantees.
+* Compare the different ways to inde a Strings by using `s3[i]` and `s3.chars.nth(i).unwrap()` where `i` is in-bound, out-of-bounds, and "on" the flag unicode character.
 
 </details>