Add basic README.

README.md

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+# MediaGoblin
+
+A basic container for a MediaGoblin media server (https://mediagoblin.org/), including Nginx and uWSGI. Assumes a separate PostgreSQL database for persistence, and an email relay host for sending email notifications.
+
+Source at https://projects.torsion.org/witten/mediagoblin-docker
+
+## Environment variables
+
+`NOTIFICATION_EMAIL`: From address for MediaGoblin notification emails.
+`EMAIL_RELAY_HOST`: Hostname used to send outgoing notification emails. Since MediaGoblin isn't terribly configurable as an email client, it's recommended that this "host" is a mail relay like https://hub.docker.com/r/alterrebe/postfix-relay/.
+`DATABASE_URL`: Connection string for database to use for persistence, including credentials. See example below.
+
+## Volumes
+
+`/app/user_dev`: Location to store user media uploads.
+
+## Ports
+
+`80`: Only listens on port 80 for HTTP. Intended to be used in conjunction with an external reverse proxy like https://hub.docker.com/r/jwilder/nginx-proxy/ to provide HTTPS or anything else that's needed.
+
+## Example
+
+```console
+docker run --detach --name=mediagoblin \
+    --env NOTIFICATION_EMAIL=media@example.org \
+    --env EMAIL_RELAY_HOST=relay.example.org \
+    --env DATABASE_URL=postgresql://mediagoblin:password@database.example.org/mediagoblin \
+    --volume /var/lib/mediagoblin/user_dev:/app/user_dev \
+    --publish 80:80 \
+    witten/mediagoblin:latest
+```