Updating README.md according to the refactoring of vtr_reg_nightly

Author: ArashAhmadian

vtr_flow/tasks/regression_tests/README.md

@@ -73,24 +73,27 @@ Create golden results with:
 <scripts_path>/parse_vtr_task.py -create_golden -l <tasks_path>/regression_tests/vtr_reg_strong/task_list.txt
 ```
 
-## LEVEL THREE  - Nightly VTR Regression - `vtr_reg_nightly_test#`
+## LEVEL THREE  - Nightly VTR Regression - `vtr_reg_nightly_test#, #:1-3` 
 
  * To be run by automated build system every night and on every pull request.
- * To keep the wall-clock time of this suite under ~4 hours using -j8, it is divided into multiple sub-suites, and each of them are submitted as different jobs to different kokoro machines. 
- * Estimated Runtime: ~15-20 hours
-
+ * To keep the wall-clock time of this suite under ~6 hours using -j8, it is divided into multiple sub-suites, and each of them are submitted as different jobs to different kokoro machines. 
+ * Estimated runtime: 30-35 hours
+ 
 DO-IT-ALL COMMAND - This command will execute, parse, and check results.
 ```
-./run_reg_test.py vtr_reg_nightly_test#
+./run_reg_test.py vtr_reg_nightly_test1
+./run_reg_test.py vtr_reg_nightly_test2
+./run_reg_test.py vtr_reg_nightly_test3
 ./run_reg_test.py vtr_reg_valgrind
 ```
+**The below commands concern a single sub-suite (# is the sub-suite number). They have to be repeated for all sub-suites to cover all tests under Nightly VTR Regression**
 
 To create golden results, use:
 ```
 ./run_reg_test.py -create_golden vtr_reg_nightly_test#
 ```
 
-Execute with:
+Execute  a sub-suite with:
 ```
 <scripts_path>/run_vtr_task.py -l <tasks_path>/regression_tests/vtr_reg_nightly_test#/task_list.txt
 ```