Update contribute_to_paddle.md (#248)

doc/build/contribute_to_paddle.md

@@ -99,3 +99,7 @@ git pull --rebase upstream HEAD
 git push -f origin HEAD
 ```
 Now your Pull Request is updated with the latest version.
+
+## Revise your pull request
+
+When you revise your pull request according to reviewer's comments, please use 'git commit' instead of 'git commit --amend' to commit your changes so that the reviewers can see the difference between the new pull requrest and the old pull request.