Fix #770: Document how to use Redis over unix sockets

changes/changelog.d/770.doc

@@ -0,0 +1 @@
+Document how to use Redis over unix sockets (#770)

deploy/env.prod.sample

@@ -75,7 +75,15 @@ REVERSE_PROXY_TYPE=nginx
 #  CACHE_URL=redis://:password@localhost:6379/0
 #  (the extra semicolon is important)
 # Use the next one if you followed Debian installation guide
+#
 # CACHE_URL=redis://127.0.0.1:6379/0
+#
+# If you want to use Redis over unix sockets, you'll actually need two variables:
+# For the cache part:
+#  CACHE_URL=redis:///run/redis/redis.sock?db=0
+# For the Celery/asynchronous tasks part:
+#  CELERY_BROKER_URL=redis+socket:///run/redis/redis.sock?virtual_host=0
+
 
 # Where media files (such as album covers or audio tracks) should be stored
 # on your system?