docs: document new mainFields option

README.md

@@ -25,20 +25,28 @@ export default {
   name: 'MyModule',
   plugins: [
     resolve({
+
+      // the fields to scan in a package.json to determine the entry point..
+      mainFields: ['module', 'main'], // Default: ['module', 'main']
+
+      // DEPRECATED: use `mainFields` instead
       // use "module" field for ES6 module if possible
       module: true, // Default: true
 
+      // DEPRECATED: use `mainFields` instead
       // use "jsnext:main" if possible
       // legacy field pointing to ES6 module in third-party libraries,
       // deprecated in favor of "pkg.module":
       // - see: https://github.com/rollup/rollup/wiki/pkg.module
       jsnext: true,  // Default: false
 
+      // DEPRECATED: use `mainFields` instead
       // use "main" field or index.js, even if it's not an ES6 module
       // (needs to be converted from CommonJS to ES6
       // – see https://github.com/rollup/rollup-plugin-commonjs
       main: true,  // Default: true
 
+      // DEPRECATED: use `mainFields` instead
       // some package.json files have a `browser` field which
       // specifies alternative files to load for people bundling
       // for the browser. If that's you, use this option, otherwise