docs: add README.md for @wdio/electron-pkg-builder

packages/@wdio_electron-pkg-builder/README.md

@@ -1,4 +1,4 @@
-# WDIO Electron Types
+# WDIO Electron Package Builder
 
 <a href="https://www.npmjs.com/package/@wdio/electron-pkg-builder" alt="NPM Version">
   <img src="https://img.shields.io/npm/v/@wdio/electron-pkg-builder" /></a>
@@ -7,4 +7,43 @@
 
 <br />
 
+## Description
+
+This package is creating rollup configuration for building up javascript code.
+The configuration to build esm and cjs code will be created by this package.  
+The `input` of Rollup option is auto detected based `exports` field of `package.json`.
+
+## How to use
+
+1. Add this library to the `devDependencies`.
+
+1. Create the configuration file for rollup `rollup.config.ts`
+
+   ```ts
+   import { createRollupConfig } from './src/index';
+
+   export default createRollupConfig();
+   ```
+
+## How to maintain rollup configuration
+
+There are 2 options.
+
+1. The common parameter between each packages managed by this monorepo
+   Please update `src/config.ts`. There is all rollup configuration include automatically generated.
+
+1. The package specific parameter
+   Please set the parameters for `createRollupConfig()` in `rollup.config.ts` in the package that needs to be changed.
+
+### Parameters
+
+If the rollup parameters is necessary to customize depend on package requirements, use following parameters to change it.
+
+|    parameter    | default                                                                                               | overview                                  |
+| :-------------: | ----------------------------------------------------------------------------------------------------- | ----------------------------------------- |
+|     rootDir     | `process.cwd()`                                                                                       | The root directory for the package.       |
+|     srcDir      | `src`                                                                                                 | The root directory for input source code. |
+|  rollupOptions  | The parameter for rollup. The parameter which is set by this library is override.                     |
+| compilerOptions | The parameter for Typescript compiler options. The parameter which is set by this library is override |
+
 **Types for the [WDIO Electron Service](https://github.com/webdriverio-community/wdio-electron-service)**