refactor: Revamp docs to separate template writers/devs

docs/installation.rst

@@ -0,0 +1,29 @@
+Installation
+============
+
+Swift Package Mangaer
+---------------------
+
+If you're using the Swift Package Manager, you can add ``Stencil`` to your
+dependencies inside ``Package.swift``.
+
+.. code-block:: swift
+
+    import PackageDescription
+
+    let package = Package(
+      name: "MyApplication",
+      dependencies: [
+        .Package(url: "https://github.com/kylef/Stencil.git", majorVersion: 0, minor: 7),
+      ]
+    )
+
+CocoaPods
+---------
+
+If you're using CocoaPods, you can add Stencil to your ``Podfile`` and then run
+``pod install``.
+
+.. code-block:: ruby
+
+    pod 'Stencil'