manual: Add a section about Freedoom's true goal. (#1094)

I've seen multiple Youtube videos recently that talk about Freedoom as
though the "free" part in the name refers to price. It's an
understandable misconception but it deserves to be corrected. Let's add
a short section to the manual to discuss this.

manual/manual.adoc

@@ -1,7 +1,9 @@
 = Freedoom Manual
 // SPDX-License-Identifier: BSD-3-Clause
 
-Welcome to Freedoom, a complete game that is free and open source software.
+Welcome to Freedoom, a complete game that is
+https://www.gnu.org/philosophy/free-sw.html[free]
+and https://opensource.org/osd/[open source software].
 Freedoom is made available under the modified BSD license, meaning that
 anyone is free to share it, modify it and reuse parts of it.
 
@@ -690,15 +692,54 @@ number of cheats that you can turn to:
 
 <<<
 
+== Freedoom is about freedom ==
+
+When people hear about Freedoom, they often assume the name refers to price --
+that the project just aims to provide an alternative to Doom that can be
+obtained for free. But this isn't the case.
+
+The word "free" has two different meanings in English. We say "free" to mean
+that something costs nothing, but we also use it to refer to freedom -- like
+"free speech" or "the free press". Freedoom is about the latter. That might
+sound confusing. What does it mean?
+
+Imagine a world where artists could only buy paints from a single company. A
+monopoly like that would mean paints would probably be more expensive, but the
+price wouldn't be the main concern. The bigger issue would be the power that it
+would grant to that company. The freedom of those artists to express themselves
+would depend on the company supplying them their paints.
+
+For over 30 years now, the Doom modding community has produced thousands upon
+thousands of levels, mods and even entirely new games built upon the original
+Doom games. These are works of art and ought to be recognised as such.
+https://www.youtube.com/watch?v=KxYND6K6u8w[Doom is an art scene].
+The raw material these works of art are made from is not paint or ink, but the
+original game itself -- endlessly modified, reused and remixed into new
+variations.
+
+The authors of Doom, id Software, have historically been very generous to the
+Doom community. From the time of the game's release they went out of their way
+to share technical details with fans, and they later released Doom's source
+code under a free software license -- something that was unknown in the games
+industry at the time and should be commended. But despite this benevolence,
+they have always held a position of power. Today, rather than being a small
+independent studio, they and the Doom franchise are owned by a large
+multinational corporation.
+
+Everybody deserves to be able to experience the wonder that is Doom and take
+part in its vibrant modding community that has endured for so many years. But
+that community also deserves its freedom and independence. By providing a free
+alternative that anyone can play, share, modify and reuse, we hope that's
+something that Freedoom can help to provide.
+
 == Contributing to Freedoom ==
 
-Freedoom is a
-https://www.gnu.org/philosophy/free-sw.html[free content]
-project contributed to by many users around the world. It is available as
-both free in cost (free as in free beer) and in modification and
-redistribution rights (free as in free speech) to end users,
-provided that the original software license is included and/or
-viewable by users of modified or redistributed versions.
+Freedoom is a https://www.gnu.org/philosophy/free-sw.html[free content]
+project contributed to by many users around the world. It is available as both
+free in cost (free as in free beer) and in modification and redistribution
+rights (free as in free speech) to end users, provided that the original
+software license is included and/or viewable by users of modified or
+redistributed versions.
 
 If you’d like to contribute to the Freedoom project, please check out the
 https://github.com/freedoom/freedoom[project’s page],