Add missing unsafe features

Author: matthewjasper

src/unsafety.md

@@ -8,4 +8,7 @@ Rust:
 
 - Dereferencing a [raw pointer](types.html#pointer-types).
 - Reading or writing a [mutable static variable](items.html#mutable-statics).
+- Reading a field of a [`union`](items.html#unions), or writing to a field of a
+  union that isn't [`Copy`](the-copy-trait.html).
 - Calling an unsafe function (including an intrinsic or foreign function).
+- Implementing an unsafe trait.