Add readme.md

readme.md

+# Omeka-S Docker Image
+## What is Omeka?
+
+Omeka provides open-source web publishing platforms for sharing digital collections and creating media-rich online exhibits.
+
+## How to use this image?
+
+This image has no database included.
+
+### Environment variables
+You can adjust the configuration of the Omeka S instane by passing variables on the `docker run` command line.
+
+`OMEKA_DB`
+This variable is mandatory and allows you to specify the name of the database for Omeka.
+
+`OMEKA_USER`, `OMEKA_PWD` 
+These variables are mandatory and specify the user who has the permissions on the database.
+
+`OMEKA_HOST`
+This variable is mandatory and specify the host for the database.
+
+## Docker Compose
+
+There is also an example of docker-compose.yml file which can be used.
+It creates 3 containers :
+- Mariadb database
+- PhpMyAdmin
+- Omeka-s without modules or themes