modify readme

Former-commit-id: d288d897945c04151691bfac0dce0fdbf4d9c81a

cpp/README.md

@@ -105,13 +105,20 @@ please reinstall CMake with curl:
    ```
 
 ##### code format and linting
-
+Install clang-format and clang-tidy
 ```shell
 CentOS 7:   
 $ yum install clang
-Ubuntu 16.04 or 18.04: 
-$ sudo apt-get install clang-format clang-tidy
-    
+Ubuntu 16.04: 
+$ sudo apt-get install clang-tidy
+$ wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
+$ sudo apt-add-repository "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-6.0 main"
+$ sudo apt-get update
+$ sudo apt-get install clang-format-6.0
+Ubuntu 18.04: 
+$ sudo apt-get install clang-tidy clang-format
+```  
+```shell
 $ ./build.sh -l
 ```
 
@@ -122,13 +129,14 @@ $ ./build.sh -u
 ```
 
 ##### Run code coverage
-
+Install lcov
 ```shell
 CentOS 7:   
 $ yum install lcov
 Ubuntu 16.04 or 18.04: 
 $ sudo apt-get install lcov
-    
+``` 
+```shell  
 $ ./build.sh -u -c
 ```