Instructions for update pull request (#84)

Author: emailweixu

doc/build/contribute_to_paddle.md

@@ -25,9 +25,12 @@ repo or just head straight to the command line:
 
 ```shell
 # Clone your fork to your local machine
-git clone [email protected]:USERNAME/Paddle.git
+git clone https://github.com/USERNAME/Paddle.git
+```
+Then you can start to develop by making a local developement branch
+```shell
+git checkout -b MY_COOL_STUFF_BRANCH origin/master
 ```
-Then you can start to develop. 
 
 ## Commit
 
@@ -45,7 +48,7 @@ are the details if any.
 
 ## Keeping Fork Up to Date
 
-Before pull your request, you shold sync you code from the latest PaddlePaddle.
+Before pull your request, you should sync your code from the latest PaddlePaddle.
 To do this, you'll need to add a remote at first:
 
 ```shell
@@ -60,8 +63,7 @@ git remote -v
 Update your fork with the latest upstream changes:
 
 ```shell
-git fetch upstream
-git pull upstream master
+git pull --rebase upstream HEAD
 ```
 
 If there are no unique commits locally, git will simply perform a fast-forward.
@@ -74,10 +76,26 @@ Now, your local master branch is up-to-date with everything modified upstream.
 
 ```shell
 # push to your repository in Github
-git push origin master
+git push origin HEAD
 ```
 
 ## Pull Request
 
 Go to the page for your fork on GitHub, select your development branch,
 and click the **pull request button**.
+
+## Update your pull request with the lastest version
+
+During the code review, your pull request may become stale because new commits in
+baidu/Paddle. GitHub allows autmotic update if there is no conflict. You can do this
+by clicking the "Update Branch" button in your pull request page. However, in the case
+of conflict, you need to do the update manually. You need to do the following on
+your local repository:
+```shell
+git checkout MY_COOL_STUFF_BRANCH
+git pull --rebase upstream HEAD
+# You may need to resolve the conflict according to the git prompt.
+# Make and test your code.
+git push -f origin HEAD
+```
+Now your Pull Request is updated with the latest version.