[docs] add document for running BR tests locally (#7044)

tests/scripts/thread-cert/border_router/README.md

@@ -0,0 +1,25 @@
+# OpenThread Border Router Tests
+
+## Run Border Router (BR) tests locally
+
+BR tests run in isolated Docker network and containers, so a new OTBR Docker image needs to be created before running these tests:
+
+```shell
+# Use root privilege when necessary.
+
+# Download OpenThread's branch of wireshark. Run this for the first time.
+./script/test get_thread_wireshark
+
+# Clear current OpenThread directory (remember to add new source files).
+git clean -xfd
+
+# Rebuild the OTBR Docker image if OTBR source code is updated.
+LOCAL_OTBR_DIR=$HOME/ot-br-posix ./script/test build_otbr_docker
+
+# Build simulated OpenThread firmware.
+VIRTUAL_TIME=0 ./script/test build
+
+# Run the BR tests locally.
+TEST_CASE=./tests/scripts/thread-cert/border_router/test_advertising_proxy.py
+VERBOSE=1 PACKET_VERIFICATION=1 VIRTUAL_TIME=0 ./script/test cert_suite ${TEST_CASE}
+```