add hyperlink to github pages for access functions api in readme

README.md

@@ -104,6 +104,17 @@ The ROS message files are auto-generated based on the [ASN.1 definitions](https:
   -o etsi_its_msgs/etsi_its_cam_msgs/msg
 ```
 
+#### Access Functions Documentation
+
+[Click here](https://ika-rwth-aachen.github.io/etsi_its_messages) to be taken to the API documentation for the implemented access functions.
+
+The documentation can be generated by running [Doxygen](https://doxygen.nl/).
+
+```bash
+# etsi_its_messages/doc$
+doxygen
+```
+
 ### `etsi_its_coding`
 
 The `etsi_its_coding` metapackage includes one dedicated package for each ETSI ITS message type, e.g., `etsi_its_cam_coding`. These packages provide C++ libraries containing a `struct` implementation of the ETSI ITS message types including functions for encoding and decoding the structures to binary buffers.