chore(docs): Add installation instructions for Carthage

Closes #27
Closes #65

docs/installation.rst

@@ -27,3 +27,26 @@ If you're using CocoaPods, you can add Stencil to your ``Podfile`` and then run
 .. code-block:: ruby
 
     pod 'Stencil'
+
+Carthage
+--------
+
+.. note:: Use at your own risk. We don't offer support for Carthage and instead recommend you use Swift Package Manager.
+
+1) Add ``Stencil`` to your ``Cartfile``:
+
+    .. code-block:: text
+
+        github "kylef/Stencil" ~> 0.7.0
+
+2) Checkout your dependencies, generate the Stencil Xcode project, and then use Carthage to build Stencil:
+
+    .. code-block:: shell
+
+        $ carthage update
+        $ (cd Carthage/Checkouts/Stencil && swift package generate-xcodeproj)
+        $ carthage build
+
+3) Follow the Carthage steps to add the built frameworks to your project.
+
+To learn more about this approach see `Using Swift Package Manager with Carthage <https://fuller.li/posts/using-swift-package-manager-with-carthage/>`_.