add install command for python3-dev

adding require commands for installing python3 development package on Ubuntu and CentOs .

Author: MahdiZareie

README.md

@@ -19,6 +19,12 @@ You may need to install the Python and MySQL development headers and libraries l
 
 On Windows, there are binary wheel you can install without MySQLConnector/C or MSVC. 
 
+#### Note on Python 3 : if you are using python3 then you need to install python3-dev using the following command :
+
+`sudo apt-get install python3-dev` # debian / Ubuntu
+
+`sudo yum install python3-devel `  # Red Hat / CentOS
+
 
 ### Install from PyPI