Updated README.md as per issue #288 (#289)

* Updated README.md as per issue #288

* Added steps to install nixFlakes explicitly, ref #289

README.md

@@ -22,23 +22,25 @@ This is an *unofficial* PDF version of "Category Theory for Programmers" by Bart
 Building
 --------
 
-The best way to build the book is using the [Nix](https://nixos.org/nix/) package manager. After installing Nix, type `nix-shell` in the root directory of the project. This will download all the needed dependencies and tools to build the book (TeXLive, required fonts and packages, Pygments theme for syntax highligting, etc.)
-
-When the download is complete, and you're prompted with a shell, use the instructions below to build the book.
-
-The `src` directory contains the LaTeX sources. To recompile the book, go there and enter:
+The best way to build the book is using the [Nix](https://nixos.org/nix/) package manager. After [installing Nix](https://nixos.org/download.html), if you're using a non-NixOS operating system, you need to install `nixFlakes` in your environment following the steps below ([source](https://nixos.wiki/wiki/Flakes#Non-NixOS)):
 
 ```bash
-$ make
+$ nix-env -iA nixpkgs.nixFlakes
 ```
 
-To build the Scala edition, type:
+Edit either `~/.config/nix/nix.conf` or `/etc/nix/nix.conf` and add:
 
-```bash
-$ make scala
 ```
+experimental-features = nix-command flakes
+```
+
+This is needed to expose the Nix 2.0 CLI and flakes support that are hidden behind feature-flags.
+
+Also, if the Nix installation is in multi-user mode, don’t forget to restart the nix-daemon. 
+
+Afterwards, type `nix flake show` in the root directory of the project to see all the available versions of this book. Then type `nix build .#<edition>` to build the edition you want (Haskell, Scala, OCaml, Reason and their printed versions). For example, to build the Scala edition you'll have to type `nix build .#ctfp-scala`.
 
-Upon successful compilation, the files will be placed in the `out` directory next to `src`. 
+Upon successful compilation, the PDF file will be placed in the `result` directory inside the root directory `milewski-ctfp-pdf` of the repository. 
 
 The file `preamble.tex` contains all the configuration and style declarations.