Update instructions to run

webpack.config.core.js

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+/**
+ * Copyright (c) 2019 The xterm.js authors. All rights reserved.
+ * @license MIT
+ */
+
+const path = require('path');
+
+/**
+ * This webpack config does a production build for xterm-core.js. It works by taking the output from tsc
+ * (via `yarn watch` or `yarn prebuild`) which are put into `xterm-core/` and webpacks them into a
+ * production mode commonjs library module in `lib/`. The aliases are used fix up the absolute paths
+ * output by tsc (because of `baseUrl` and `paths` in `tsconfig.json`.
+ */
+module.exports = {
+  entry: './xterm-core/common/public/Terminal.js',
+  devtool: 'source-map',
+  module: {
+    rules: [
+      {
+        test: /\.js$/,
+        use: ["source-map-loader"],
+        enforce: "pre",
+        exclude: /node_modules/
+      }
+    ]
+  },
+  resolve: {
+    modules: ['./node_modules'],
+    extensions: [ '.js' ],
+    alias: {
+      common: path.resolve('./xterm-core/common')
+    }
+  },
+  output: {
+    filename: 'xterm-core.js',
+    path: path.resolve('./lib'),
+    libraryTarget: 'commonjs'
+  },
+  mode: 'production',
+  target: 'node',
+};