Add link to README.md

Add link to published documentation.

Author: Amanda Butler

README.md

@@ -33,3 +33,7 @@ We have a [developer website](https://os.mbed.com) for asking questions, engagin
 ## Getting started for contributors
 
 We also have a [contributing and publishing guide](https://os.mbed.com/contributing/) that covers licensing, contributor agreements and style guidelines.
+
+## Documentation
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+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.