(doc) less technical headline, more compact introduction

* FAQ: moving motivation from README.
* README: headline is more appealing to a non-technical public. Introduction now presents WebTorrent and ActivityPub.

FAQ.md

@@ -5,6 +5,7 @@
 <!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
 
 
+- [Why did you create PeerTube?](#why-did-you-create-peertube)
 - [I don't like the name "PeerTube"](#i-dont-like-the-name-peertube)
 - [If nobody watches a video, is it seeded?](#if-nobody-watches-a-video-is-it-seeded)
 - [What is WebSeed?](#what-is-webseed)
@@ -22,6 +23,28 @@
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 
+## Why did you create PeerTube?
+
+We can't build a FOSS video streaming alternative to YouTube, Dailymotion,
+Vimeo... with centralized software. One organization alone may not have
+enough money to pay for bandwidth and video storage of its servers.
+
+Our stance is that only a decentralized network of servers can provide an
+acceptable answer to technical issues (bandwidth, transcoding expenses, etc.)
+and social answers (need for a particular moderation policy, preserving
+content, etc.).
+
+While a paragraph is not enough to answer all these problems, PeerTube has
+very early prouded itself for using a contributory design, both for creating
+communities as federated nodes (as [Mastodon](https://joinmastodon.org/) for
+example), and for seeding videos (instances can seed each other's videos). But it's not
+enough because one video could become popular and overload the server. That is
+why we need to use a P2P protocol to limit the server load. Thanks to
+[WebTorrent](https://github.com/feross/webtorrent), we can use BitTorrent
+inside most modern web browsers, and users become seeds as the video gets
+more viewers.
+
+
 ## I don't like the name "PeerTube"
 
 PeerTube is just the name of the software. You can install it on your