lnrpc: add SendToRoute command

lnrpc/README.md

@@ -72,6 +72,11 @@ description):
      * Send a payment over Lightning to a target peer.
   * SendPaymentSync
      * SendPaymentSync is the synchronous non-streaming version of SendPayment.
+  * SendToRoute
+    * Send a payment over Lightning to a target peer through a route explicitly
+      defined by the user.
+  * SendToRouteSync
+    * SendToRouteSync is the synchronous non-streaming version of SendToRoute.
   * AddInvoice
      * Adds an invoice to the daemon. Invoices are automatically settled once
        seen as an incoming HTLC.