Rename CachedTreeHash trait, add readme

eth2/utils/tree_hash/README.md

@@ -0,0 +1,76 @@
+# Tree hashing
+
+Provides both cached and non-cached tree hashing methods.
+
+## Standard Tree Hash
+
+```rust
+use tree_hash_derive::TreeHash;
+
+#[derive(TreeHash)]
+struct Foo {
+	a: u64,
+	b: Vec<u64>,
+}
+
+fn main() {
+	let foo = Foo {
+		a: 42,
+		b: vec![1, 2, 3]
+	}
+
+	println!("root: {}", foo.tree_hash_root());
+}
+```
+
+## Cached Tree Hash
+
+
+```rust
+use tree_hash_derive::{TreeHash, CachedTreeHash};
+
+#[derive(TreeHash, CachedTreeHash)]
+struct Foo {
+	a: u64,
+	b: Vec<u64>,
+}
+
+#[derive(TreeHash, CachedTreeHash)]
+struct Bar {
+	a: Vec<Foo>,
+	b: u64,
+}
+
+fn main() {
+	let bar = Bar {
+		a: vec![
+			Foo {
+				a: 42,
+				b: vec![1, 2, 3]
+			}
+		],
+		b: 42
+	};
+
+	let modified_bar = Bar {
+		a: vec![
+			Foo {
+				a: 100,
+				b: vec![1, 2, 3, 4, 5, 6]
+			}
+			Foo {
+				a: 42,
+				b: vec![]
+			}
+		],
+		b: 99
+	};
+
+
+    let mut hasher = CachedTreeHasher::new(&bar).unwrap();
+	hasher.update(&modified_bar);
+
+	// Assert that the cached tree hash matches a standard tree hash.
+	assert_eq!(hasher.tree_hash_root(), modified_bar.tree_hash_root());
+}
+```