nixpkgs manual: various improvements in introduction

- rename to preface to be inline with NixOS manual
- refer to other manuals
- use https
- change example channel to 19.09

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+---
+title: Preface
+author: Frederik Rietdijk
+date: 2015-11-25
+---
+
+# Preface
+
+The Nix Packages collection (Nixpkgs) is a set of thousands of packages for the
+[Nix package manager](https://nixos.org/nix/), released under a
+[permissive MIT/X11 license](https://github.com/NixOS/nixpkgs/blob/master/COPYING).
+Packages are available for several platforms, and can be used with the Nix
+package manager on most GNU/Linux distributions as well as [NixOS](https://nixos.org/nixos).
+
+This manual primarily describes how to write packages for the Nix Packages collection
+(Nixpkgs). Thus it’s mainly for packagers and developers who want to add packages to
+Nixpkgs. If you like to learn more about the Nix package manager and the Nix
+expression language, then you are kindly referred to the [Nix manual](https://nixos.org/nix/manual/).
+The NixOS distribution is documented in the [NixOS manual](https://nixos.org/nixos/manual/).
+
+## Overview of Nixpkgs
+
+Nix expressions describe how to build packages from source and are collected in
+the [nixpkgs repository](https://github.com/NixOS/nixpkgs). Also included in the
+collection are Nix expressions for
+[NixOS modules](https://nixos.org/nixos/manual/index.html#sec-writing-modules).
+With these expressions the Nix package manager can build binary packages.
+
+Packages, including the Nix packages collection, are distributed through
+[channels](https://nixos.org/nix/manual/#sec-channels). The collection is
+distributed for users of Nix on non-NixOS distributions through the channel
+`nixpkgs`. Users of NixOS generally use one of the `nixos-*` channels, e.g.
+`nixos-19.09`, which includes all packages and modules for the stable NixOS
+19.09. Stable NixOS releases are generally only given
+security updates. More up to date packages and modules are available via the
+`nixos-unstable` channel.
+
+Both `nixos-unstable` and `nixpkgs` follow the `master` branch of the Nixpkgs
+repository, although both do lag the `master` branch by generally
+[a couple of days](https://howoldis.herokuapp.com/). Updates to a channel are
+distributed as soon as all tests for that channel pass, e.g.
+[this table](https://hydra.nixos.org/job/nixpkgs/trunk/unstable#tabs-constituents)
+shows the status of tests for the `nixpkgs` channel.
+
+The tests are conducted by a cluster called [Hydra](http://nixos.org/hydra/),
+which also builds binary packages from the Nix expressions in Nixpkgs for
+`x86_64-linux`, `i686-linux` and `x86_64-darwin`.
+The binaries are made available via a [binary cache](https://cache.nixos.org).
+
+The current Nix expressions of the channels are available in the
+[`nixpkgs`](https://github.com/NixOS/nixpkgs) repository in branches
+that correspond to the channel names (e.g. `nixos-19.09-small`).