add docs from PR #854 to fabio.properties

fabio.properties

@@ -634,12 +634,21 @@
 #       "cmd": "string",
 #       "service": "string",
 #       "src": "string",
-#       "dest": "string",
+#       "dst": "string",
 #       "weight": float,
 #       "tags": ["string"],
 #       "opts": {"string":"string"}
 #     }
 #   ]
+# Short description of the fields required for a custom backend
+#
+# - cmd - the command to add, remove or change weight of a route. For example `route add` to add a new route mapping.
+# - service - the name that the service will show up in the UI.
+# - src - usually the prefix that will be used in the routing table.
+# - dst - the endpoint that will be used as the destination of the routing table.
+# - weight - defines the weight of this path to perform routing. For example route A 90% and route B 10% for canary deployments.
+# - tags - a list of tags, provide a way to filter routes, making it easier to do operations like bulk deletes `route del tags "dev"`.
+# - opts - a KV map of the config language list of options. for example `proto` or `prefix`
 #
 # The default is
 #