documented "preferred_ranges" (#541)

Document the preferred config variable, and deprecate "local_range".

examples/config.yml

@@ -112,9 +112,11 @@ punchy:
 # IMPORTANT: this value must be identical on ALL NODES/LIGHTHOUSES. We do not/will not support use of different ciphers simultaneously!
 #cipher: chachapoly
 
-# Local range is used to define a hint about the local network range, which speeds up discovering the fastest
+# Preferred ranges is used to define a hint about the local network ranges, which speeds up discovering the fastest
 # path to a network adjacent nebula node.
-#local_range: "172.16.0.0/24"
+# NOTE: the previous option "local_range" only allowed definition of a single range
+# and has been deprecated for "preferred_ranges"
+#preferred_ranges: ["172.16.0.0/24"]
 
 # sshd can expose informational and administrative functions via ssh this is a
 #sshd: