Update installation instruction using Linux package managers

Added NixOS instruction. Close #1731

README.md

@@ -26,11 +26,13 @@ Table of Contents
       * [Using Homebrew or Linuxbrew](#using-homebrew-or-linuxbrew)
       * [Using git](#using-git)
       * [As Vim plugin](#as-vim-plugin)
-      * [Arch Linux](#arch-linux)
-      * [Debian](#debian)
-      * [Fedora](#fedora)
-      * [openSUSE](#opensuse)
-      * [FreeBSD](#freebsd)
+      * [Using Linux package managers](#using-linux-package-managers)
+        * [Arch Linux](#arch-linux)
+        * [Debian](#debian)
+        * [Fedora](#fedora)
+        * [FreeBSD](#freebsd)
+        * [NixOS](#nixos)
+        * [openSUSE](#opensuse)
       * [Windows](#windows)
    * [Upgrading fzf](#upgrading-fzf)
    * [Building fzf](#building-fzf)
@@ -141,13 +143,15 @@ Plug 'junegunn/fzf', { 'dir': '~/.fzf', 'do': './install --all' }
   " and you don't have to run the install script if you use fzf only in Vim.
 ```
 
-### Arch Linux
+### Using Linux package managers
+
+#### Arch Linux
 
 ```sh
 sudo pacman -S fzf
 ```
 
-### Debian
+#### Debian
 
 fzf is available in Debian Buster and above, and can be installed using the usual
 method:
@@ -156,9 +160,10 @@ method:
 sudo apt-get install fzf
 ```
 
-Read the documentation (/usr/share/doc/fzf/README.Debian) on how to enable it.
+Read the documentation (`/usr/share/doc/fzf/README.Debian`) on how to enable
+it.
 
-### Fedora
+#### Fedora
 
 fzf is available in Fedora 26 and above, and can be installed using the usual
 method:
@@ -169,20 +174,28 @@ sudo dnf install fzf
 
 Shell completion and plugins for vim or neovim are enabled by default. Shell
 key bindings are installed but not enabled by default. See Fedora's package
-documentation (/usr/share/doc/fzf/README.Fedora) for more information.
+documentation (`/usr/share/doc/fzf/README.Fedora`) for more information.
 
-### openSUSE
+#### FreeBSD
 
-fzf is available in openSUSE Tumbleweed and can be installed via zypper:
+```sh
+pkg install fzf
+```
+
+#### NixOS
+
+You can use the [Nix package manager](https://nixos.org/nix/) to install fzf.
 
 ```sh
-sudo zypper install fzf
+nix-env -iA nixpkgs.fzf
 ```
 
-### FreeBSD
+#### openSUSE
+
+fzf is available in openSUSE Tumbleweed and can be installed via zypper:
 
 ```sh
-pkg install fzf
+sudo zypper install fzf
 ```
 
 ### Windows