Added a readme!

Author: James O'Cull

README.md

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+# And Rust said, "Let there be units."
+
+Hurray! Now you can work with units of measure in a headache-free way.
+
+Currently only `Length` is available, but I am planning to add more shortly.
+
+### Example
+
+```
+let football_field = Length::from_yards(1.0);
+let meters = football_field.as_meters();
+println!("There are {} meters in a football field.", meters);
+```