Update string-slices.md (#209)

* Update string-slices.md

* Adding an extra example of converting `String` back to `&str`.
* C++ who would often read this guide could get immediate intuitive understanding of Rust strings if you compare them to C++ equivalents.

* Expand on `String` vs `std::string`

Co-authored-by: Martin Geisler <martin@geisler.net>

src/basic-syntax/string-slices.md

@@ -4,13 +4,16 @@ We can now understand the two string types in Rust:
 
 ```rust,editable
 fn main() {
-    let s1: &str = "Hello";
+    let s1: &str = "World";
     println!("s1: {s1}");
 
     let mut s2: String = String::from("Hello ");
     println!("s2: {s2}");
     s2.push_str(s1);
     println!("s2: {s2}");
+    
+    let s3: &str = &s2[6..];
+    println!("s3: {s3}");
 }
 ```
 
@@ -23,10 +26,17 @@ Rust terminology:
 
 * `&str` introduces a string slice, which is an immutable reference to UTF-8 encoded string data stored in a block of memory. String literals (`”Hello”`), are stored in the program’s binary.
 
-* Rust’s `String` type is a wrapper around a vector of bytes. As with a `Vec<T>`, it is mutable and owned.
+* Rust’s `String` type is a wrapper around a vector of bytes. As with a `Vec<T>`, it is owned.
+    
+* As with many other types 
 
 * `String::from` creates a string from a string literal; `String::new` creates a new empty string, to which string data can be added using the `to_string` method.
 
 * The `push_str` method appends a string slice to the string.
     
+* You can borrow `&str` slices from `String` via `&` and optionally range selection.
+    
+* For C++ programmers: think of `&str` as `const char*` from C++, but the one that always points 
+  to a valid string in memory. Rust `String` is a rough equivalent of `std::string` from C++ (main difference: it can only contain UTF-8 encoded bytes and will never use a small-string optimization).
+    
 </details>