Merge pull request #3746 from goldeagle/doc/fix-plugin-install-with-cmake

upd: README.md with submodule contents of go & grafana connectors

README.md

@@ -83,12 +83,18 @@ sudo dnf install -y maven
 
 ## Get the source codes
 
-- github:
+First of all, you may clone the source codes from github:
 ```bash
 git clone https://github.com/taosdata/TDengine.git
 cd TDengine
 ```
 
+The connectors for go & grafana have been moved to separated repositories,
+so you should run this command in the TDengine directory to install them:
+```bash
+git submodule update --init --recursive
+```
+
 ## Build TDengine
 
 ### On Linux platform