Documentation: add Hello World and How to add it to Giraffe (#245)

* Documentation: add Hello World and How to add it to Giraffe

* documentation: adding saturn to giraffe

* docs: add links to explanations

* doc: link to SAFE Dojo from getting started

Co-authored-by: Krzysztof Cieślak <krzysztof_cieslak@windowslive.com>

Author: 0x53A

docs/content/tutorials/adding-pages.md

@@ -1,7 +1,7 @@
 ---
 title: Adding Pages
 category: tutorial
-menu_order: 2
+menu_order: 3
 ---
 
 # Adding Pages

docs/content/tutorials/adding-saturn-to-an-existing-giraffe-app.md

@@ -0,0 +1,77 @@
+---
+title: Getting started
+category: tutorial
+menu_order: 2
+---
+
+# Adding Saturn to an existing Giraffe application
+
+The previous chapter showed how to get started with a new application. 
+
+If you already have a working Giraffe webserver, you can gradually opt-in to Saturn.
+
+For example, if your existing app looks like this:
+
+```f#
+
+type Customer = {
+    Name : string
+    Address : string
+}
+
+let customers =
+    choose [
+      GET >=> (json { Name = "Mr. Smith"; Address = "Santa Monika"})
+      PUT >=> (bindJson<Customer> (fun customer -> printfn "Adding customer %A" customer; setStatusCode 200))
+    ]
+
+
+let webApp =
+    choose [
+        route "/"               >=> htmlFile "/pages/index.html"
+        route "/api/customers"   >=> customers
+    ]
+```
+
+and you need to add "vendor" functionality, you could implement it as a Saturn ``router`` while keeping everything else intact:
+
+
+```f#
+// the new Saturn router
+let vendors = router {
+    getf "/%i" (fun vendorId -> (json (readVendorFromDb vendorId)))
+    post "/" (bindJson<Vendor> (fun customer -> addVendor vendor; setStatusCode 200))
+}
+
+let webApp =
+    choose [
+        route "/"                >=> htmlFile "/pages/index.html"
+        route "/api/customers"   >=> customers
+        // plug the new Saturn router into the Giraffe app
+        route "/api/vendors"     >=> vendors
+    ]
+```
+
+# Embedding Giraffe Handlers into Saturn
+
+Of course the other way around also works.
+
+For example, [Elmish.Bridge](https://github.com/Nhowka/Elmish.Bridge) does not provide a specialized implementation for Saturn. And it doesn't need to, because we can just use the Giraffe implementation!
+
+```f#
+
+open Elmish
+open Elmish.Bridge
+
+let elmishBridgeHandler : HttpHandler =
+  Bridge.mkServer Shared.endpoint init update
+  |> Bridge.run Giraffe.server
+
+// our existing Saturn router
+let router = router {
+
+    // ...
+
+    forward "" elmishBridgeHandler
+}
+```

docs/content/tutorials/how-to-start.md

@@ -10,6 +10,9 @@ Requirements:
 
 * `dotnet` SDK 3.1 [https://dotnet.microsoft.com/download/dotnet-core/3.1](https://dotnet.microsoft.com/download/dotnet-core/3.1)
 
+## Template
+
+The easiest way to get started is to use the provided template:
 
 1. Install the `dotnet` template with `dotnet new -i Saturn.Template`
 2. Create a new folder and move into it - `mkdir SaturnSample && cd SaturnSample`
@@ -20,3 +23,31 @@ Requirements:
 8. Open the folder in your favourite editor (VSCode) and insert the line (`forward "/books" Books.Controller.resource`) into `browserRouter` in `Router.fs` file
 9. Start the application by running `dotnet fake build -t run` from the root of the solution. This will start the application in watch mode (automatic recompilation on changes) and open your browser on [http://localhost:8085](http://localhost:8085) which should display the index page.
 10. Go to [http://localhost:8085/books](http://localhost:8085/books) to see the generated view. All buttons should be working; you can add new entries, and remove or edit old ones.
+
+
+## Hello World
+
+If you want to start from scratch with a minimal Saturn webserver:
+
+1. Create a new F# Project (for example with `dotnet new console -lang F#`)
+2. Add the `Saturn` NuGet Package
+
+```f#
+open Saturn
+open Giraffe
+
+let app = application {
+    use_router (text "Hello World from Saturn")
+}
+
+run app
+```
+
+If you compile and run this application, it will unconditionally return the text regardless of the path.
+
+From here on out you can add [routers](../explanations/routing.md), [controllers](../explanations/controller.md) and [views](../explanations/view.md).
+
+## Deep Dive
+
+To get a better understanding of Saturn and the whole SAFE Stack you can look at the [SAFE-Dojo](https://github.com/CompositionalIT/SAFE-Dojo).
+