cmd/rc: add --json flag for structured JSON input

docs/content/rc.md

@@ -67,6 +67,25 @@ $ rclone rc rc/noop param1=one param2=two
 Run `rclone rc` on its own to see the help for the installed remote
 control commands.
 
+`rclone rc` also supports a `--json` flag which can be used to send
+more complicated input parameters.
+
+```
+$ rclone rc --json '{ "p1": [1,"2",null,4], "p2": { "a":1, "b":2 } }' rc/noop
+{
+	"p1": [
+		1,
+		"2",
+		null,
+		4
+	],
+	"p2": {
+		"a": 1,
+		"b": 2
+	}
+}
+```
+
 ## Supported commands
 <!--- autogenerated start - run make rcdocs - don't edit here -->
 ### cache/expire: Purge a remote from cache