added workspaces example

the_book/workspaces/add_one/src/lib.rs

@@ -0,0 +1,12 @@
+pub fn add_one(x: i32) -> i32 {
+    x + 1
+}
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_works() {
+        assert_eq!(3, add_one(2))
+    }
+}