chore(mariadb): add role-README, update collection README

roles/mariadb/README.md

@@ -0,0 +1,19 @@
+# `finallycoffee.base.mariadb` ansible role
+
+This role deploys a MariaDB instance in a docker container.
+
+## Usage
+
+The role expects the following variables to be populated with values and/or secrets:
+
+```yaml
+mariadb_root_password: #mariadb root password
+mariadb_database: # name of the database to create
+mariadb_username: # name of a user to auto-create and assign permission on the mariadb_database
+mariadb_password: # password of the user in mariadb_username
+```
+
+## Requirements
+
+- Docker installed
+- python-docker present on target system for ansible to be able to talk with the docker API.