apache config: add instructions for mediaproxy.

installation/pleroma-apache.conf

 # default Apache site config for Pleroma
 #
 # needed modules: define headers proxy proxy_http proxy_wstunnel rewrite ssl
+# optional modules: cache cache_disk
 #
 # Simple installation instructions:
 # 1. Install your TLS certificate, possibly using Let's Encrypt.
@@ -8,6 +9,14 @@
 # 3. This assumes a Debian style Apache config. Copy this file to
 #    /etc/apache2/sites-available/ and then add a symlink to it in
 #    /etc/apache2/sites-enabled/ by running 'a2ensite pleroma-apache.conf', then restart Apache.
+#
+# Optional: enable disk-based caching for the media proxy
+# For details, see https://git.pleroma.social/pleroma/pleroma/wikis/How%20to%20activate%20mediaproxy
+#
+# 1. Create the directory listed below as the CacheRoot, and make sure
+#    the Apache user can write to it.
+# 2. Configure Apache's htcacheclean to clean the directory periodically.
+# 3. Run 'a2enmod cache cache_disk' and restart Apache.
 
 Define servername example.tld
 
@@ -34,6 +43,15 @@ CustomLog ${APACHE_LOG_DIR}/access.log combined
     SSLCompression          off
     SSLSessionTickets       off
 
+    # uncomment the following to enable mediaproxy caching on disk
+    # <IfModule mod_cache_disk.c>
+    #     CacheRoot /var/cache/apache2/mod_cache_disk
+    #     CacheDirLevels 1
+    #     CacheDirLength 2
+    #     CacheEnable disk /proxy
+    #     CacheLock on
+    # </IfModule>
+
     RewriteEngine On
     RewriteCond %{HTTP:Connection} Upgrade [NC]
     RewriteCond %{HTTP:Upgrade} websocket [NC]