Add license generation

rollup.config.js

@@ -2,6 +2,7 @@
 
 import commonjs from '@rollup/plugin-commonjs'
 import { nodeResolve } from '@rollup/plugin-node-resolve'
+import license from 'rollup-plugin-license'
 
 const config = {
   input: 'src/index.js',
@@ -9,9 +10,18 @@ const config = {
     esModule: true,
     file: 'dist/index.js',
     format: 'es',
-    sourcemap: false
+    sourcemap: true
   },
-  plugins: [commonjs(), nodeResolve()]
+  plugins: [
+    commonjs(),
+    nodeResolve({ preferBuiltins: true }),
+    license({
+      sourcemap: true,
+      thirdParty: {
+        output: 'dist/licenses.txt'
+      }
+    })
+  ]
 }
 
 export default config