tests: Bluetooth: Bsim tests: expand README a bit

Expand a bit the README file to cover the compile.sh and
run_parallel.sh scripts, in case users would like to use
them locally, to guide them a bit.

Signed-off-by: Alberto Escolar Piedras <[email protected]>

Author: aescolar

tests/bluetooth/bsim_bt/README.txt

@@ -1,2 +1,11 @@
 This folder contains tests meant to be run with BabbleSim's physical layer
-simulation, and therefore cannot be run directly from sanitycheck
+simulation, and therefore cannot be run directly from sanitycheck
+
+The compile.sh and run_parallel.sh scripts are used by the CI system to build
+the needed images and execute these tests in batch.
+
+You can also run them manually if desired, but be sure to call them setting
+the variables they expect. For example, from Zephyr's root folder, you can run:
+
+WORK_DIR=${ZEPHYR_BASE}/bsim_bt_out tests/bluetooth/bsim_bt/compile.sh
+RESULTS_FILE=${ZEPHYR_BASE}/myresults.xml SEARCH_PATH=tests/bluetooth/bsim_bt/bsim_test_app/tests_scripts tests/bluetooth/bsim_bt/run_parallel.sh