Update README.md

FIX centos 环境下编译YCM同样需要cmake

Author: losingle

README.md

@@ -90,6 +90,7 @@ molokai主题
         sudo yum groupinstall 'Development Tools'
         sudo rpm -Uvh http://download.fedoraproject.org/pub/epel/7/x86_64/e/epel-release-7-5.noarch.rpm
         sudo yum install the_silver_searcher
+        sudo yum install cmake
 
         # mac
         brew install ctags