WIP: update the quickstart according to Xin's comment #38.

docs/quickstart.md

@@ -7,12 +7,28 @@ Start the container in lxslc7 (OS: CentOS7):
 $ /cvmfs/container.ihep.ac.cn/bin/hep_container shell SL6
 ```
 
-## Get code using git
+## Manage code using git
 
+Fork the CEPCSW into your own repo. For an example:
+* https://github.com/*USERNAME*/CEPCSW
+
+Get the source code from your own repo:
 ```
-$ git clone git@github.com:cepc/CEPCSW.git
+$ git clone git@github.com:USERNAME/CEPCSW.git
 $ cd CEPCSW
 ```
+
+Add the upstream repo:
+```
+$ git remote add cepc https://github.com/cepc/CEPCSW.git
+```
+
+Sync and merge the upstream repo:
+```
+$ git remote update
+$ git merge cepc/master
+```
+
 ## Setup and build 
 
 ```