add submodule instructions in build_and_install doc

Author: wangyang59

doc/getstarted/build_and_install/build_from_source_en.md

@@ -14,6 +14,16 @@ cd paddle
 git submodule update --init --recursive
 ```
 
+If you already have a local PaddlePaddle repo and have not initialized the submodule, you can simply run the following command in your PaddlePaddle home directory.
+```
+git submodule update --init --recursive
+```
+
+To sync with the upstream submodule repo, you can run the following command
+```
+git submodule update --remote
+```
+
 ## <span id="requirements">Requirements</span>
 
 To compile the source code, your computer must be equipped with the following dependencies.