Docs for writing an adapter (#1846)

Author: benmccann

documentation/docs/10-adapters.md

@@ -33,14 +33,13 @@ export default {
 
 A variety of official adapters exist for serverless platforms...
 
-- [`adapter-begin`](https://github.com/sveltejs/kit/tree/master/packages/adapter-begin) — for [Begin](https://begin.com)
 - [`adapter-cloudflare-workers`](https://github.com/sveltejs/kit/tree/master/packages/adapter-cloudflare-workers) — for [Cloudflare Workers](https://developers.cloudflare.com/workers/)
 - [`adapter-netlify`](https://github.com/sveltejs/kit/tree/master/packages/adapter-netlify) — for [Netlify](https://netlify.com)
 - [`adapter-vercel`](https://github.com/sveltejs/kit/tree/master/packages/adapter-vercel) — for [Vercel](https://vercel.com)
 
-...and others:
+...and traditional platforms:
 
 - [`adapter-node`](https://github.com/sveltejs/kit/tree/master/packages/adapter-node) — for creating self-contained Node apps
 - [`adapter-static`](https://github.com/sveltejs/kit/tree/master/packages/adapter-static) — for prerendering your entire site as a collection of static files
 
-> The adapter API is still in flux and will likely change before 1.0.
+As well as [community-provided adapters](https://github.com/sveltejs/integrations#sveltekit-adapters). You may also [write your own adapter](#writing-an-adapter).

documentation/docs/80-adapter-api.md

@@ -0,0 +1,34 @@
+---
+title: Writing an Adapter
+---
+
+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 packages must implement the following API, which creates an `Adapter`:
+```
+/**
+ * @param {AdapterSpecificOptions} options
+ */
+export default function (options) {
+	/** @type {import('@sveltejs/kit').Adapter} */
+	return {
+		name: '',
+		async adapt({ utils, config }) {
+		}
+	};
+}
+```
+
+The types for `Adapter` and its parameters are available in [types/config.d.ts](https://github.com/sveltejs/kit/blob/master/packages/kit/types/config.d.ts).
+
+Within the `adapt` method, there are a number of things that an adapter should do:
+- Clear out the build directory
+- Output code that:
+  - Calls `init`
+  - Converts from the patform's request to a [SvelteKit request](#hooks-handle), calls `render`, and converts from a [SvelteKit response](#hooks-handle) to the platform's
+  - Globally shims `fetch` to work on the target platform. SvelteKit provides a `@sveltejs/kit/install-fetch` helper for platforms that can use `node-fetch`
+- Bundle the output to avoid needing to install dependencies on the target platform, if desired
+- Call `utils.prerender`
+- Put the user's static files and the generated JS/CSS in the correct location for the target platform
+
+> The adapter API may change before 1.0.