Added some basic documentation for length module

Author: James O'Cull

src/length/mod.rs

@@ -1,8 +1,9 @@
 pub mod consts;
 mod traits;
-
 use self::consts::*;
 
+/// The `Length` struct can be used to deal with lengths in a common way.
+/// Common metric and imperial units are supported.
 #[derive(Copy, Clone, Debug)]
 pub struct Length {
     meters: f64

src/lib.rs

@@ -1,5 +1,16 @@
 #[cfg(test)]
 mod tests;
 
+/// The `Length` struct can be used to deal with lengths in a common way.
+/// Common metric and imperial units are supported.
+/// 
+/// # Example
+/// 
+/// ```
+/// let football_field = Length::from_yards(1.0);
+/// let meters = football_field.as_meters();
+/// println!("There are {} meters in a football field.", meters);
+/// ```
+
 #[allow(dead_code)]
 pub mod length;