Added an example to the readme.

README.md

@@ -69,6 +69,7 @@ NOTE: This entire README can be markdown linted with
 <!-- toc -->
 
 - [Overview](#overview)
+  - [Example](#example)
 - [Get in touch](#get-in-touch)
 - [License and copyright](#license-and-copyright)
 
@@ -84,6 +85,14 @@ cftime calendar functionality.
 
 Documentation can be found at <https://scitools.org.uk/cf_units/docs/latest/>.
 
+### Example
+
+    >>> from cf_units import Unit
+    >>> km = Unit('kilometers')
+    >>> m = Unit('meters')
+    >>> m.convert(1500, km)
+    1.5
+
 ## Get in touch
 
 - Questions, ideas, general discussion or announcements