add instructions for MSLIBS_CACHE_PATH

resource/faq/FAQ_en.md

@@ -45,6 +45,12 @@ A: MindSpore imports third party dependencies through submodule mechanism, among
 
 <br/>
 
+Q: How to change installation directory of the third party libraries?
+
+A: The third party libraries will be installed in build/mindspore/.mslib, you can change the installation directory by setting the environment variable MSLIBS_CACHE_PATH, eg. `export MSLIBS_CACHE_PATH = ~/.mslib`.
+
+<br/>
+
 Q: What should I do if the software version required by MindSpore is not the same with the Ubuntu default software version ?
 
 A: At the moment some software might need manual upgrade. (**Note**：MindSpore requires Python3.7.5 and gcc7.3，the default version in Ubuntu 16.04 are Python3.5 and gcc5，whereas the one in Ubuntu 18.04 are Python3.7.3 and gcc7.4)

resource/faq/FAQ_zh_cn.md

@@ -44,6 +44,12 @@ A：MindSpore通过submodule机制引入第三方依赖包，其中`protobuf`依
 
 <br/>
 
+Q：如何改变第三方依赖库安装路径？
+
+A：第三方依赖库的包默认安装在build/mindspore/.mslib目录下，可以设置环境变量MSLIBS_CACHE_PATH来改变安装目录，比如 `export MSLIBS_CACHE_PATH = ~/.mslib`。
+
+<br/>
+
 Q：MindSpore要求的配套软件版本与Ubuntu默认版本不一致怎么办？
 
 A：当前MindSpore只提供版本配套关系，需要您手动进行配套软件的安装升级。（**注明**：MindSpore要求Python3.7.5和gcc7.3，Ubuntu 16.04默认为Python3.5和gcc5，Ubuntu 18.04默认自带Python3.7.3和gcc7.4）