add a section about step4, submit a pull request

HOWTOCONTRIBUTE.md

@@ -24,6 +24,8 @@ When you implement a sample program of an algorithm, please keep the following i
 ### Step 3: Implement unit test
 If you added a new sample program, please add a unit test code for it under test directory. When you implement the test code, please refer to the existing code. Additionally, each unit tests should be able to run without animation because test process stops and an exception occurs. After you completed to implement the test code, confirm the test passes by executing the script, run_test_suites.sh/bat.  
 
+### Step 4: Submit a pull request and modify code based on review
+
 ## Report and fix defect
 
 ## Add documentation about existing program