Update README.md (#41)

diff --git a/README.md b/README.md
index 52951a9..22eef04 100644
--- a/README.md
+++ b/README.md
@@ -21,6 +21,8 @@
 ```shell
 git clone https://github.com/apache/skywalking-cli
 cd skywalking-cli
+git submodule init
+git submodule update
 make
 ```