Add GH link to README.md

Add link for contributing to docs, as well.

Author: Amanda Butler

README.md

@@ -37,3 +37,5 @@ We also have a [contributing and publishing guide](https://os.mbed.com/contribut
 ## Documentation
 
 For more information about Mbed OS, please see [our published documentation](https://os.mbed.com/docs/latest). It includes published Doxygen for our APIs, step-by-step tutorials, porting information and background reference materials about our architecture and tools.
+
+To contribute to this documentation, please see the [mbed-os-5-docs repo](https://github.com/ARMmbed/mbed-os-5-docs).