docs(readme): improve documentation

README.md

@@ -22,15 +22,15 @@ import plugins, {file} from 'rollup-plugin-by-output';
 export default {
   // ...
   plugins: plugins(babel(), [file(pkg.browser), terser()]),
-  output: {
-    globals: {
-      lodash: '_'
-    },
-    name: pkg.name,
-    file: pkg.browser,
-    format: 'umd'
-  },
   output: [
+    {
+      globals: {
+        lodash: '_'
+      },
+      name: pkg.name,
+      file: pkg.browser,
+      format: 'umd'
+    },
     {
       file: pkg.main,
       format: 'cjs'
@@ -44,6 +44,73 @@ export default {
 
 ```
 
+## What
+
+Rollup support multiple outputs with the same input. But sometime we want to apply different plugins for these outputs.
+The most common scenario is apply the [terser](https://github.com/TrySound/rollup-plugin-terser) plugin for a minify bundle.
+Before we can write a config array, but there are a lot of duplicate code and operations.
+
+This plugin (maybe not a plugin) gives you a slightly elegant and efficient solution for this scene.
+
+## APIs
+
+```js
+import plugins, {when, whenAll, prop, format, file} from 'rollup-plugin-by-output';
+```
+
+### plugins
+
+```js
+{
+  //...
+  plugins: [pluginA, when(filter, pluginB), pluginsC, ..., ...whenAll(anotherFilter, pluginD, pluginE)]
+}
+```
+
+same as:
+
+```js 
+{
+  // ...
+  plugins: plugins(pluginA,
+    [filter, pluginB],// same as when
+    pluginsC, ..., 
+    [anotherFilter, pluginD, pluginE] // same as whenAll and flat
+  )
+}
+```
+
+The filter is a predicate function, the parameter is an output config object. If the filter return a truthy value, the rest plugins will be apply for the output.
+
+`when` and `whenAll` is convenient for few filters and `plugins` is convenient for multiple filters.
+
+### filter helpers
+
+There are three [simple](https://github.com/wmzy/rollup-plugin-by-output/blob/master/src/index.js#L53) but useful filter helpers: `prop`, `format`, `file`.
+
+With them you can write like:
+```js 
+// output 
+[
+  {
+    name: pkg.name,
+    file: pkg.browser,
+    format: 'umd'
+    }
+]
+
+when(format('umd'), pluginA)
+// same as
+when(format(/^umd$/), pluginA)
+// same as
+when(format(f => f === 'umd'), pluginA)
+
+when(file(pkg.browser), pluginA)
+
+format = filter => prop('format', filter)
+file = filter => prop('file', filter)
+```
+
 ## Workflow
 
 ```bash
@@ -62,3 +129,6 @@ npm run commit
 # publish
 npm publish
 ```
+## License
+
+MIT