add readme to metadata-relay

metadata_relay/README.md

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+# MetadataRelay
+
+This is a service that provides metadata for movies and TV shows. It caches the metadata to not overload the TMDB and
+TVDB APIs. This service is used by MediaManager to fetch metadata for movies and TV shows. I (the developer) run a
+public instance of this service at https://metadata-relay.maxid.me, but you can also run your
+own instance.
+
+## Example Docker Compose Configuration
+
+````yaml
+services:
+  valkey:
+    image: valkey/valkey:latest
+    restart: always
+    container_name: valkey
+    expose:
+      - 6379
+  metadata_relay:
+    image: ghcr.io/maxdorninger/mediamanager/metadata_relay:latest
+    restart: always
+    environment:
+      - TMDB_API_KEY=  # you need not provide a TMDB API key, if you only want to use TVDB metadata, or the other way around
+      - TVDB_API_KEY=
+      - VALKEY_HOST=valkey
+    container_name: metadata_relay
+    ports:
+      - 8000:8000
+````