add basic readme

Author: timhaywood

README.md

@@ -0,0 +1,50 @@
+# expression-globals-typescript
+
+This library mocks the After Effects expressions API in TypeScript, providing types and the available global functions and variables.
+
+> To be used with [rollup-plugin-ae-jsx](https://www.npmjs.com/package/rollup-plugin-ae-jsx), which will convert the code to After Effects friendly `.jsx` files and remove the globals imported from this package.
+
+## Usage
+
+1. Install
+
+   ```sh
+   npm install expression-globals-typescript --save-dev
+   ```
+
+   Found in our [Expressions Library Template](https://github.com/motiondeveloper/expressions-library-template).
+
+2. Import globals functions as needed
+
+   ```js
+   import {
+     timeToFrames,
+     add,
+     length,
+     random,
+   } from "expression-globals-typescript";
+   ```
+
+3. Import object bases to create layer, comp, and property mocks
+
+   ```js
+   import {
+     PropertyBase,
+     LayerBase,
+     CompBase,
+   } from "expression-globals-typescript";
+
+   const thisProperty = Object.create(PropertyBase);
+   const thisLayer = Object.create(LayerBase);
+   const thisComp = Object.create(CompBase);
+   ```
+
+   You can then use the properties and methods of these objects as you would within expressions.
+
+## Why?
+
+When writing expressions in **Typescript** using our [Expressions Library Template](https://github.com/motiondeveloper/expressions-library-template) and [rollup-plugin-ae-jsx](https://www.npmjs.com/package/rollup-plugin-ae-jsx), TypeScript will complain if you try to use the global functions and objects available in expressions.
+
+This is because they aren't defined in the development environment.
+
+`expression-globals-typescript` solves this by mocking the entire expressions API in TypeScript, so you can import and use these globals, with the advantages of them being fully typed.