clarify typical installation steps

README.md

@@ -19,7 +19,8 @@ A Babel plugin which automatically makes stack traces source-map aware
 
 # INSTALL
 
- npm install babel-plugin-source-map-support
+ npm install source-map-support
+ npm install babel-plugin-source-map-support --save-dev
 
 # SYNOPSIS
 
@@ -64,7 +65,7 @@ but it no-ops if it has already been loaded, so there is no harm in registering
 
 You probably don't want to use this plugin when compiling code for the web because you probably don't
 want to include inline source maps in minified code. An easy way to limit the plugin's use to
-development/test builds, is to use Babel's [`env` option](https://babeljs.io/docs/usage/babelrc/#env-option) e.g.:
+development/test builds is to use Babel's [`env` option](https://babeljs.io/docs/usage/babelrc/#env-option) e.g.:
 
 ```javascript
 {