added function and variable lessons.

the_book/functions/src/main.rs

@@ -0,0 +1,16 @@
+fn main() {
+    another_function(5);
+    print_labeled_measurement(50, 'm');
+    println!("The returned value is {}", plus_one(4));
+}
+
+fn another_function(x: i32) {
+    println!("The value of x is: {}", x);
+}
+fn print_labeled_measurement(value: i32, unit_label: char) {
+    println!("The measurement is: {}{}", value, unit_label);
+}
+//adds one and returns
+fn plus_one(x:i32) -> i32 {
+    x+1
+}