Update updated documentation section

Add link to updated contributing guide.

Author: Amanda Butler

docs/introduction/introduction.md

@@ -83,6 +83,7 @@ Please see the following sections of our documentation for more information.
 
 ### Recently updated documentation
 
+* An updated [contributing guide](../contributing/index.html).
 * A revised [porting guide](../porting/index.html).
 * A [review of the full process of taking a project to production](./evaluation-production/index.html).
 * An updated [quick start](../quick-start/index.html) to demonstrate more Mbed OS capabilities.