Update index.md

_chiron/guides/index.md

@@ -10,6 +10,42 @@ Chiron works rather differently to most .NET JSON libraries with which you might
 
 We're still working on a full set of guides to Chiron, but there is already writing out there -- see the [Links][links] section.
 
+## Reading and writing Json values
+
+Chiron is split into `Json<_>` which is for reading and writing JSON and `Json` which is the object model for JSON.
+
+Let's start by writing a discriminated union serialiser:
+
+```fsharp
+type DU =
+  | A
+  | B
+// serialiser
+open Chiron
+open Chiron.Operators
+open Aether
+open Aether.Operators
+let serialise (d: DU): Json<unit> =
+  match d with
+  | A ->
+    Json.Optic.set Json.String_ "a"
+  | B ->
+    Json.Optic.set Json.String_ "b"
+```
+
+Similarly, we can write the deserialiser:
+
+```fsharp
+let deserialise _: Json<DU> =
+      function
+      | "a" -> A
+      | _   -> B
+  <!> Json.Optic.get Json.String_
+```
+
+That's the very basics. TODO: writing code for fatter objects and choosing to either keep the serialiser inside or outside of the type declaration.
+
+
 <!--- Local --->
 
 [aether]: /aether