add brief docs

README.md

@@ -1,5 +1,34 @@
 # rollup-plugin-cheap-ts
 
+A hacky lightweight Rollup plugin for transpiling and bundling libraries written in TypeScript.
+
+## Installation
+
+This is not published to npm. Install from Git tags.
+
+## Usage
+
+Include the plugin in your `rollup.config.js`:
+
+```javascript
+import cheapTS from 'rollup-plugin-cheap-ts';
+
+export default {
+	// ...
+	plugins: [
+		// ...
+		cheapTS(),
+		// ...
+	],
+};
+```
+
+You must have a `tsconfig.json` in the same directory as your `rollup.config.js`. This file must specify an `"include": []` array, as this is used when calling the `tsc` binary so it knows which `.ts` files to compile.
+
+All references to TypeScript files must be extensionless, and all extensionless imports must be TypeScript files.
+
+This probably doesn't work on Windows.
+
 ## License
 
 Copyright (c) 2018 Conduitry