Docs for writing an adapter API

documentation/docs/80-adapter-api.md

@@ -0,0 +1,27 @@
+---
+title: Writing Adapters
+---
+
+We recommend [looking at the source for an adapter](https://github.com/sveltejs/kit/tree/master/packages) to a platform similar to yours and copying it as a starting point.
+
+Adapters must implement the following API:
+```
+export default function () {
+	/** @type {import('@sveltejs/kit').Adapter} */
+	return {
+		name: '',
+		async adapt({ utils, config }) {
+		}
+	};
+}
+```
+
+Within the `adapt` method, there are a few things an adapter should do:
+- Clear out the build directory
+- Provide code that runs:
+  - Call `init`
+  - Convert from the patform's request to a SvelteKit request, call `render`, convert from a SveteKit reponse to the platform's
+- Adapters typically bundle the output to avoid needing to install dependencies on the target platform, etc.
+- Globally shim `fetch` to work on the target platform. SvelteKit provides a `@sveltejs/kit/install-fetch` helper to use `node-fetch`
+- Call `prerender`
+- Put the user's static files and the generated JS/CSS in the correct location for the target platform