Reference E2 agent tutorial in doc/README.md

doc/README.md

@@ -43,6 +43,7 @@ There is some general information in the [OpenAirInterface Gitlab Wiki](https://
 - [How to run OAI-VNF and OAI-PNF](./RUN_NR_NFAPI.md) _Note: does not work currently_
 - [How to use the positioning reference signal (PRS)](./RUN_NR_PRS.md)
 - [How to use device-to-device communication (D2D, 4G)](./d2d_emulator_setup.txt)
+- [How to run with E2 agent](../openair2/E2AP/README.md)
 
 Legacy unmaintained files:
 - `L2NFAPI_NOS1.md`, `L2NFAPI_S1.md`: old L2simulator, not valid anymore