doc: add how_to_create_a_pull.md

docs/howto/how_to_create_a_pull.md

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+How to create a pull request
+==========================
+
+You can follow the standard [github approach](https://help.github.com/articles/using-pull-requests/) to contribute code. Here is one sample setup.
+
+- Fork a new repo with your GitHub username.
+- Set up your github personal email and user name
+
+```
+git config user.name "XXX"
+git config user.email "XXX@[XXX.com]"
+```
+
+- Add Apollo repository as upstream
+
+```
+git remote add upstream [email protected]:ApolloAuto/apollo.git
+```
+
+- Create a new branch, make changes and commit.
+
+```
+git checkout -b  "my_dev"
+```
+
+- Sync up with the Apolloauto/apollo repo
+
+```
+git pull --rebase  upstream master
+```
+
+- Push local developments to your own forked repository
+
+```
+git push -f -u origin "my_dev"
+```
+
+- Generate a new pull request between "Apolloauto/apollo:master" and "forked repo:my_dev"
+- Collaborators will review and merge the commit.
+
+Thanks a lot for your contributions! 
+