Add type definition file

package.json

@@ -3,6 +3,7 @@
   "version": "3.0.0",
   "description": "Autoload Config for PostCSS",
   "main": "src/index.js",
+  "types": "types/index.d.ts",
   "files": [
     "src"
   ],

types/index.d.ts

@@ -0,0 +1,44 @@
+// Type definitions for postcss-load-config 2.1
+import Processor from 'postcss/lib/processor'
+import { Plugin, ProcessOptions, Transformer } from "postcss";
+import { Options as CosmiconfigOptions } from 'cosmiconfig';
+
+// In the ConfigContext, these three options can be instances of the
+// appropriate class, or strings. If they are strings, postcss-load-config will
+// require() them and pass the instances along.
+interface ProcessOptionsPreload {
+    parser?: string | ProcessOptions['parser'];
+    stringifier?: string | ProcessOptions['stringifier'];
+    syntax?: string | ProcessOptions['syntax'];
+}
+
+// The remaining ProcessOptions, sans the three above.
+type RemainingProcessOptions =
+    Pick<ProcessOptions, Exclude<keyof ProcessOptions, keyof ProcessOptionsPreload>>;
+
+// Additional context options that postcss-load-config understands.
+interface Context {
+    cwd?: string;
+    env?: string;
+}
+
+// The full shape of the ConfigContext.
+type ConfigContext = Context & ProcessOptionsPreload & RemainingProcessOptions;
+
+// Result of postcssrc is a Promise containing the filename plus the options
+// and plugins that are ready to pass on to postcss.
+type ResultPlugin = Plugin | Transformer | Processor;
+
+interface Result {
+    file: string;
+    options: ProcessOptions;
+    plugins: ResultPlugin[];
+}
+
+declare function postcssrc(ctx?: ConfigContext, path?: string, options?: CosmiconfigOptions): Promise<Result>;
+
+declare namespace postcssrc {
+    function sync(ctx?: ConfigContext, path?: string, options?: CosmiconfigOptions): Result;
+}
+
+export = postcssrc;