started book again

book/hello_cargo/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "hello_cargo"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

book/hello_cargo/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("Hello, world!");
+}