Add installation as a separate header (#3251)

Installation was part of quick start and it was not easy to spot.

docs/00-introduction.md

@@ -6,9 +6,15 @@ title: Introduction
 
 Rollup is a module bundler for JavaScript which compiles small pieces of code into something larger and more complex, such as a library or application. It uses the new standardized format for code modules included in the ES6 revision of JavaScript, instead of previous idiosyncratic solutions such as CommonJS and AMD. ES modules let you freely and seamlessly combine the most useful individual functions from your favorite libraries. This will eventually be possible natively everywhere, but Rollup lets you do it today.
 
+### Installation
+
+```console 
+npm install --global rollup
+```
+
 ### Quick start
 
-Install with `npm install --global rollup`. Rollup can be used either through a [command line interface](guide/en/#command-line-reference) with an optional configuration file, or else through its [JavaScript API](guide/en/#javascript-api). Run `rollup --help` to see the available options and parameters.
+ Rollup can be used either through a [command line interface](guide/en/#command-line-reference) with an optional configuration file, or else through its [JavaScript API](guide/en/#javascript-api). Run `rollup --help` to see the available options and parameters.
 
 > See [rollup-starter-lib](https://github.com/rollup/rollup-starter-lib) and
 [rollup-starter-app](https://github.com/rollup/rollup-starter-app) to see