RXSwift: add library via Carthage

This library is added by Carthage in a dedicated build folder that is
contained in the Carthage folder.

The Cartfile.resolved file is tracked by Git, so a Carthage bootstrap
will be needed to download the frameworks.

Tuleap: #1377
Change-Id: I8df4efdd6488cd73201df80b327b5f00f2b2a21f
diff --git a/Ring/Cartfile.resolved b/Ring/Cartfile.resolved
new file mode 100644
index 0000000..e6f82ff
--- /dev/null
+++ b/Ring/Cartfile.resolved
@@ -0,0 +1 @@
+github "ReactiveX/RxSwift" "3.0.1"