Add notes on Trait Extensione & extern keyword

Rust/Rust.md

@@ -662,6 +662,25 @@ fn generic_method<T, U>()
 fn generic_return() -> impl RequiredTrait { }
 ```
 
+### Trait Extensions
+
+```rs
+extern crate foo;
+use foo::Foo;
+
+// define the methods to be added to the existing type
+trait FooExt {
+    fn bar(&self);
+}
+
+// implement the extension methods
+impl FooExt for Foo {
+    fn bar(&self) { .. }
+}
+```
+
+**NOTE**: the calling code needs to import your new trait in addition to the external type
+
 ## Lifetimes
 
 Lifetime annotation indicates to the *borrow checker* that the lifetime of the returned value is as long as the lifetime of the referenced value.  
@@ -889,3 +908,34 @@ fs::write("path/to/file", contents);
 let mut file = fs::OpenOptions::new().append(true).open("path/to/file").unwrap();
 file.write(b"appended text");  // write wants an [u8]
 ```
+
+## External Code
+
+The extern keyword is used in two places in Rust:
+
+- in conjunction with the crate keyword to make Rust code aware of other Rust crates in the project
+- in foreign function interfaces (FFI).
+
+`extern` is used in two different contexts within FFI. The first is in the form of external blocks, for declaring function interfaces that Rust code can call foreign code by.
+
+```rs
+#[link(name = "my_c_library")]
+extern "C" {
+    fn my_c_function(x: i32) -> bool;
+}
+```
+
+This code would attempt to link with `libmy_c_library.so` on unix-like systems and `my_c_library.dll` on Windows at runtime, and panic if it can't find something to link to.  
+Rust code could then use my_c_function as if it were any other unsafe Rust function.  
+Working with non-Rust languages and FFI is inherently unsafe, so wrappers are usually built around C APIs.
+
+The mirror use case of FFI is also done via the extern keyword:
+
+```rs
+#[no_mangle]
+pub extern "C" fn callable_from_c(x: i32) -> bool {
+    x % 3 == 0
+}
+```
+
+If compiled as a dylib, the resulting `.so` could then be linked to from a C library, and the function could be used as if it was from any other library