add documentation: targets/ARCH/rfsimulator/README.md

targets/ARCH/rfsimulator/README.md

+#General
+This is a RF simulator that allows to test OAI without a RF board.
+It replaces a actual RF board driver.
+
+As much as possible, it works like a RF board, but not in realtime: it can run faster than realtime if there is enough CPU or slower (it is CPU bound instead of real time RF sampling bound)
+
+#build 
+You can build it the same way, and together with actual RF driver
+
+Example:
+```bash
+./build_oai --ue-nas-use-tun --UE --eNB -w SIMU
+```
+It is also possible to build actual RF and use choose on each run:
+```bash
+./build_oai --ue-nas-use-tun --UE --eNB -w USRP --rfsimulator
+```
+Will build both the eNB (lte-softmodem) and the UE (lte-uesoftmodem)
+We recommend to use the option --ue-nas-use-tun that is much simpler to use than the OAI kernel driver.
+
+#usage
+Setting the env variable RFSIMULATOR enables the RF board simulator
+It should the set to "enb" in the eNB
+
+For the UE, it should be set to the IP address of the eNB
+example: 
+```bash
+sudo RFSIMULATOR=192.168.2.200 ./lte-uesoftmodem -C 2685000000 -r 50 
+```
+Except this, the UE and the eNB can be used as it the RF is real
+
+If you reach 'RA not active' on UE, be careful to generate a valid SIM
+```bash
+$OPENAIR_DIR/targets/bin/conf2uedata -c $OPENAIR_DIR/openair3/NAS/TOOLS/ue_eurecom_test_sfr.conf -o .
+```
+#Caveacts
+Still issues in power control: txgain, rxgain are not used
+
+no S1 mode is currently broken, so we were not able to test the simulator in noS1 mode