README: document concurrent fuzzing command line

Author: boxerab

test/fuzzers/README

@@ -21,7 +21,7 @@ Build and run Locally
 ```
 python ../oss-fuzz/infra/helper.py build_fuzzers --external . --sanitizer address
 python ../oss-fuzz/infra/helper.py run_fuzzer --external . css_parse_fuzzer -- -verbosity=3 -print_final_stats=1
-python ../oss-fuzz/infra/helper.py run_fuzzer --external . --sanitizer address css_parse_fuzzer -- -verbosity=3 -print_final_stats=1 -max_total_time=6000
+python ../oss-fuzz/infra/helper.py run_fuzzer --external . --sanitizer address css_parse_fuzzer -- -verbosity=3 -print_final_stats=1 -max_total_time=6000 -jobs=16 -workers=16
 ```