Improve docs introduction section

docs/introduction.md

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+Welcome! This is the official documentation for Foris JS.
+
+## What Foris JS is
+
+Foris JS library is a set of components and utils for reForis application and
+plugins.
+
+Please notice that all of these components or utils are used in reForis and
+plugins. If you want to study them by example, I recommend you to full-text
+search those repositories.
+
+# Installation
+
+## Prerequisites
+
+Please make sure that [Node.js](https://nodejs.org/en/) is installed on your
+system.
+
+The current Long Term Support (LTS) release is an ideal starting point, see
+[here](https://github.com/nodejs/Release#release-schedule).
+
+## Installation
+
+To install the latest release:
+
+```plain
+npm install foris
+```
+
+To install a specific version:
+
+```plain
+npm install foris@version
+```
+
+<a target="_blank" href="https://www.npmjs.com/package/foris">Check
+on<img width="100px" src="./docs/forisjs-npm.svg"></a>