Add TypeScript documentation

docs/README.md

@@ -7,6 +7,7 @@
 - [Path Binding](./path-binding.md)
 - [Plugins](./plugins.md)
 - [Best Practices](./best-practices.md)
+- [TypeScript Support](./typescript.md)
 - [Wiki (Usage Examples)](https://github.com/Profiscience/ko-component-router/wiki)
 
 #### Further Reading

docs/typescript.md

@@ -0,0 +1,57 @@
+# TypeScript Support
+
+If you're using TypeScript — *as you most definitely should be* — you're in luck! The router is written with first-class support for TypeScript.
+
+That being said, it isn't immediately obvious how to take advantage of type-safety if you use middleware and plugins that extend the context.
+
+Let's say you want to have some middleware that sets a `data` property on the context, like so...
+
+```javascript
+import { Router } from 'ko-component-router'
+
+Router.use(async (ctx) => {
+  ctx.data = await fetchSomeData()
+})
+```
+
+Because `ctx` is strongly-typed, we get an error as expected...
+
+> Property 'data' does not exist on type 'Context & IContext'
+
+We could use the dirty `(ctx as any).data = ...` hack that anyone who has used TypeScript for more than a day has undoubtedly had to use, but it would be way better if we could let the compiler know about the new property so we get all the benefits type-safety brings to the table like autocompletion. To do this, it's as simple as adding a `declare` statement to our middleware file, like so...
+
+```typescript
+import { Router } from 'ko-component-router'
+
+declare module 'ko-component-router' {
+  interface IContext {
+    data?: MyDataType
+  }
+}
+
+Router.use(async (ctx) => {
+  ctx.data = await fetchSomeData()
+})
+```
+
+That's it! If `fetchSomeData()` returns something of the wrong type, the compiler will throw an error, and in your component viewModels, you will have full autocomplete of your custom properties.
+
+**NOTE:** It's an interface prefixed with `I`, *not* the normal `Context` class. This is because TypeScript does not support declaration merging on classes.
+
+You may also take advantage of some types that are exported, namely `RouteConfig`, `RouteMap`, `Middleware`, and `Plugin`. You can use these to specify types where the compiler cannot otherwise infer them. For example...
+
+```typescript
+import { Plugin } from 'ko-component-router'
+
+declare module 'ko-component-router' {
+  interface IContext {
+    data: string
+  }
+}
+
+const apiPlugin: Plugin = ({ apiUrl }) => async (ctx) => {
+  ctx.data = await $.get(apiUrl)
+}
+
+export default apiPlugin
+```