added workspaces example

the_book/workspaces/adder/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "adder"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+add_one = {path = "../add_one"}

the_book/workspaces/adder/src/main.rs

@@ -0,0 +1,6 @@
+use add_one;
+
+fn main() {
+    let num = 10;
+    println!("Hello World! {} plus 1 is {}", num, add_one::add_one(num));
+}