📝 [webpack-source-map-loader] doc: add README

packages/webpack-source-map-loader/README.md

@@ -9,3 +9,26 @@
 ```shell
 yarn add @barusu/webpack-source-map-loader
 ```
+
+## Use in webpack
+
+```javascript
+module.exports = {
+  module: {
+    rules: [
+      {
+        test: /\.js$/,
+        use: ['@barusu/webpack-source-map-loader'],
+        enforce: "pre"
+      }
+    ]
+  }
+};
+```
+
+# Why
+
+ It seems that [source-map-loader](https://github.com/webpack-contrib/source-map-loader) is no longer maintained, but there still some problems when you are using lerna to structure your project:
+
+  * Relative path used by sourcemap in package in monorepo cannot be parsed correctly in other packages
+    - see [Load all sources without sourceContent and resolve relative sources with context](https://github.com/webpack-contrib/source-map-loader/pull/91)