Add docs clarifying compatibility with MariaDB

Peewee is compatible with MariaDB Server. Add clarifying notes in the
documentation, including logo, etc.

Testing
```
docker run -d -p 3306:3306 --name mariadb_container -e MYSQL_ROOT_PASSWORD=password mariadb
```

```
>>> from peewee import MySQLDatabase
>>> database = MySQLDatabase('information_schema', user='root', password='password', host='127.0.0.1', port=3306)
>>> database.connect()
True
>>> version_query = database.execute_sql('SELECT VERSION()')
>>> print(version_query.fetchone()[0])
11.3.2-MariaDB-1:11.3.2+maria~ubu2204
>>> database.close()
True
```

README.rst

@@ -7,7 +7,7 @@ Peewee is a simple and small ORM. It has few (but expressive) concepts, making i
 
 * a small, expressive ORM
 * python 2.7+ and 3.4+
-* supports sqlite, mysql, postgresql and cockroachdb
+* supports sqlite, mysql, mariadb, postgresql and cockroachdb
 * tons of `extensions <http://docs.peewee-orm.com/en/latest/peewee/playhouse.html>`_
 
 New to peewee? These may help:

docs/index.rst

@@ -13,7 +13,7 @@ it easy to learn and intuitive to use.
 
 * a small, expressive ORM
 * python 2.7+ and 3.4+
-* supports sqlite, mysql, postgresql and cockroachdb
+* supports sqlite, mysql, mariadb, postgresql and cockroachdb
 * :ref:`tons of extensions <playhouse>`
 
 .. image:: postgresql.png
@@ -24,6 +24,10 @@ it easy to learn and intuitive to use.
     :target: peewee/database.html#using-mysql
     :alt: mysql
 
+.. image:: mariadb.png
+    :target: peewee/database.html#using-mariadb
+    :alt: mariadb
+
 .. image:: sqlite.png
     :target: peewee/database.html#using-sqlite
     :alt: sqlite

docs/peewee/database.rst

@@ -12,8 +12,8 @@ to open a connection to a database, and then can be used to:
 * Manage transactions (and savepoints).
 * Introspect tables, columns, indexes, and constraints.
 
-Peewee comes with support for SQLite, MySQL and Postgres. Each database class
-provides some basic, database-specific configuration options.
+Peewee comes with support for SQLite, MySQL, MariaDB and Postgres. Each
+database class provides some basic, database-specific configuration options.
 
 .. code-block:: python
 
@@ -557,6 +557,14 @@ If you would like to use APSW, use the :py:class:`APSWDatabase` from the
 
     apsw_db = APSWDatabase('my_app.db')
 
+.. _using_mariadb:
+
+Using MariaDB
+-----------
+
+Peewee supports MariaDB. To use MariaDB, use the MySQL backend, which is shared
+between the two. See :ref:`"Using MySQL" <using_mysql>` for more details.
+
 .. _using_mysql:
 
 Using MySQL
@@ -1773,11 +1781,11 @@ handler.
 Adding a new Database Driver
 ----------------------------
 
-Peewee comes with built-in support for Postgres, MySQL and SQLite. These
-databases are very popular and run the gamut from fast, embeddable databases to
-heavyweight servers suitable for large-scale deployments.  That being said,
-there are a ton of cool databases out there and adding support for your
-database-of-choice should be really easy, provided the driver supports the
+Peewee comes with built-in support for Postgres, MySQL, MariaDB and SQLite.
+These databases are very popular and run the gamut from fast, embeddable
+databases to heavyweight servers suitable for large-scale deployments.  That
+being said, there are a ton of cool databases out there and adding support for
+your database-of-choice should be really easy, provided the driver supports the
 `DB-API 2.0 spec <http://www.python.org/dev/peps/pep-0249/>`_.
 
 .. warning::