Add documentation on installing MySQL and newer versions on CentOS 7.

Author: geerlingguy

README.md

@@ -107,6 +107,29 @@ The rest of the settings in `defaults/main.yml` control MySQL's memory usage and
 
 Replication settings. Set `mysql_server_id` and `mysql_replication_role` by server (e.g. the master would be ID `1`, with the `mysql_replication_role` of `master`, and the slave would be ID `2`, with the `mysql_replication_role` of `slave`). The `mysql_replication_user` uses the same keys as `mysql_users`, and is created on master servers, and used to replicate on all the slaves.
 
+### Later versions of MySQL on CentOS 7
+
+If you want to install MySQL from the official repository instead of installing the system default MariaDB equivalents, you can add the following `pre_tasks` task in your playbook:
+
+```yaml
+  pre_tasks:
+    - name: Install the MySQL repo.
+      yum:
+        name: http://repo.mysql.com/mysql-community-release-el7-5.noarch.rpm
+        state: present
+      when: ansible_os_family == "RedHat"
+  
+    - name: Override variables for MySQL (RedHat).
+      set_fact:
+        mysql_daemon: mysqld
+        mysql_packages: ['mysql-server']
+        mysql_log_error: /var/log/mysqld.err
+        mysql_syslog_tag: mysqld
+        mysql_pid_file: /var/run/mysqld/mysqld.pid
+        mysql_socket: /var/lib/mysql/mysql.sock
+      when: ansible_os_family == "RedHat"
+```
+
 ### MariaDB usage
 
 This role works with either MySQL or a compatible version of MariaDB. On RHEL/CentOS 7+, the mariadb database engine was substituted as the default MySQL replacement package. No modifications are necessary though all of the variables still reference 'mysql' instead of mariadb.