carthage.md: add page (#1278)

pages/osx/carthage.md

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+# carthage
+
+> Carthage is a dependency management tool for Cocoa applications.
+
+- Download and build all dependencies mentioned in Cartfile. Also used to update dependencies to their latest version:
+
+`carthage update`
+
+- Update dependencies and only build for iOS:
+
+`carthage update --platform ios`
+
+- Update dependencies but don't build:
+
+`carthage update --no-build`
+
+- Download and rebuild the current dependency set without updating:
+
+`carthage bootstrap`
+
+- Rebuild a specific dependency:
+
+`carthage build {{dependency}}`