Update README.md (#41)

README.md

@@ -21,6 +21,8 @@ As SkyWalking CLI is using `Makefile`, compiling the project is as easy as execu
 ```shell
 git clone https://github.com/apache/skywalking-cli
 cd skywalking-cli
+git submodule init
+git submodule update
 make
 ```