Document how the build works

webpack.config.js

@@ -5,6 +5,12 @@
 
 const path = require('path');
 
+/**
+ * This webpack config does a production build for xterm.js. It works by taking the output from tsc
+ * (via `yarn watch` or `yarn prebuild`) which are put into `out/` and webpacks them into a
+ * production mode umd 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: './out/public/Terminal.js',
   resolve: {
@@ -12,7 +18,8 @@ module.exports = {
     extensions: [ '.js' ],
     alias: {
       common: path.resolve('./out/common'),
-      core: path.resolve('./out/core')
+      core: path.resolve('./out/core'),
+      ui: path.resolve('./out/ui')
     }
   },
   output: {