Add missing step in doc (#22)

Author: kezhenxu94

docs/FAQ.md

@@ -14,11 +14,13 @@ you can also disable the plugins via environment variables `SW_AGENT_DISABLE_PLU
 
 #### Q: How to build from sources?
 
-#### A: If you want to build the SkyWalking Python Agent from sources, for some reasons, for example, you want to try the latest features
-that're not released yet, please clone the source codes and `make install` it:
+#### A: If you want to build the SkyWalking Python Agent from source codes, for some reasons, for example, you want to try the latest features that're not released yet, please clone the source codes from GitHub and `make install` it:
 
 ```shell
 git clone https://github.com/apache/skywalking-python
 cd skywalking-python
+git submodule update --init
 make install
 ``` 
+
+**NOTE** that because we use Git submodule, the aforementioned commands don't apply if you download the source codes (`.tgz`) from our official website.