Add build dependency for pkg-config to README (#621)

Fix #620

README.md

@@ -71,8 +71,8 @@ support in some user forum.  Don't file a issue on the issue tracker.**
 
 You may need to install the Python 3 and MySQL development headers and libraries like so:
 
-* `$ sudo apt-get install python3-dev default-libmysqlclient-dev build-essential`  # Debian / Ubuntu
-* `% sudo yum install python3-devel mysql-devel`  # Red Hat / CentOS
+* `$ sudo apt-get install python3-dev default-libmysqlclient-dev build-essential pkg-config`  # Debian / Ubuntu
+* `% sudo yum install python3-devel mysql-devel pkgconfig`  # Red Hat / CentOS
 
 Then you can install mysqlclient via pip now: