Initial doc about storing remote media

docs/administration/storing_remote_media.md

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+# Storing Remote Media
+
+Pleroma does not store remote/federated media by default. The best way to achieve this is to change Nginx to keep its reverse proxy cache
+forever and to activate the `MediaProxyWarmingPolicy` MRF policy in Pleroma which will automatically fetch all media through the proxy
+as soon as the post is received by your instance.
+
+## Nginx
+
+We should be using `proxy_store` here I think???
+
+```
+    location ~ ^/(media|proxy) {
+        proxy_cache        pleroma_media_cache;
+        slice              1m;
+        proxy_cache_key    $host$uri$is_args$args$slice_range;
+        proxy_set_header   Range $slice_range;
+        proxy_http_version 1.1;
+        proxy_cache_valid  200 206 301 304 1h;
+        proxy_cache_lock   on;
+        proxy_ignore_client_abort on;
+        proxy_buffering    on;
+        chunked_transfer_encoding on;
+        proxy_ignore_headers Cache-Control;
+        proxy_hide_header  Cache-Control;
+        proxy_pass         http://127.0.0.1:4000;
+    }
+```
+
+## Pleroma
+
+Add to your `prod.secret.exs`:
+
+```
+config :pleroma, :instance,
+  rewrite_policy: [Pleroma.Web.ActivityPub.MRF.MediaProxyWarmingPolicy]
+```