Add basic installation instructions in the README (#25)

README.md

@@ -8,6 +8,24 @@ A web API that allowing applications to manage achievements which can be unlocke
 
 [![Website](https://img.shields.io/website?url=https%3A%2F%strawberry.ryg.one%2F)](https://strawberry.ryg.one/)
 
+## Installation
+
+Use the provided Docker image, configuring the following environment variables:
+
+`IS_DB_URI`
+: SQLAlchemy engine URL to use to select the database.
+
+`IS_WEB_HOST`
+: Host to bind the webserver to.
+
+`IS_WEB_PORT`
+: TCP port to bind the webserver to.
+
+`IS_SECRET`
+: Secret used for administration tasks.
+
+`CORS_ALLOW_ORIGINS`
+: Origins to return in the `Access-Control-Allow-Origins` header.
 
 ## Roadmap