Add license info for rtools-backports

LICENSE.txt

@@ -890,7 +890,7 @@ This project include code from Google's Asylo project.
 * cpp/src/arrow/result.h is based on status_or.h
 
 Copyright (c)  Copyright 2017 Asylo authors
-Homepage: https://asylo.dev/ 
+Homepage: https://asylo.dev/
 License: Apache 2.0
 
 ---------------------------------------------------------------------------------
@@ -900,8 +900,8 @@ This project includes code from Google's protobuf project
 * cpp/src/arrow/result.h ARROW_ASSIGN_OR_RAISE is based off ASSIGN_OR_RETURN
 
 Copyright 2008 Google Inc.  All rights reserved.
-Homepage: https://developers.google.com/protocol-buffers/ 
-License: 
+Homepage: https://developers.google.com/protocol-buffers/
+License:
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are
@@ -933,3 +933,15 @@ Code generated by the Protocol Buffer compiler is owned by the owner
 of the input file used when generating it.  This code is not
 standalone and requires a support library to be linked with it.  This
 support library is itself covered by the above license.
+
+--------------------------------------------------------------------------------
+
+This project includes code from the rtools-backports project.
+
+* ci/PKGBUILD and ci/appveyor-build-r.sh are based on code
+  from the rtools-backports project.
+
+Copyright: Copyright (c) 2013 - 2019, Алексей and Jeroen Ooms.
+All rights reserved.
+Homepage: https://github.com/r-windows/rtools-backports
+License: 3-clause BSD