Add Mysql/MariaDB support

docs/mysql-configuration.markdown

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+How to use Mysql or MariaDB instead of Sqlite
+=============================================
+
+By default Kanboard use Sqlite to stores its data.
+However it's possible to use Mysql or MariaDB instead of Sqlite.
+
+By example, it can be useful if you don't want to store any data on the web server itself.
+
+Mysql configuration
+-------------------
+
+### Create a database
+
+The first step is to create a database on your Mysql server.
+By example, you can do that with the command line mysql client:
+
+```sql
+CREATE DATABASE kanboard;
+```
+
+### Create a config file
+
+All application constants can be overrided by using a config file at the root of the project.
+The second step is to create a config file named `config.php`:
+
+```
+.
+├── LICENSE
+├── common.php
+├── ....
+├── config.php <== Our config file
+├── ....
+├── index.php
+├── ....
+└── vendor
+```
+
+### Define Mysql parameters
+
+Inside our config file write those lines:
+
+```php
+<?php
+
+// We choose to use Mysql instead of Sqlite
+define('DB_DRIVER', 'mysql');
+
+// Mysql parameters
+define('DB_USERNAME', 'REPLACE_ME');
+define('DB_PASSWORD', 'REPLACE_ME');
+define('DB_HOSTNAME', 'REPLACE_ME');
+define('DB_NAME', 'kanboard');
+```
+
+Now, you are ready to use Mysql!