smaller fixes

doc/manual/src/contributing/hacking.md

@@ -1 +1,77 @@
 # Hacking
+
+This section provides some notes on how to hack on Nix. To get the
+latest version of Nix from GitHub:
+
+```console
+$ git clone https://github.com/NixOS/nix.git
+$ cd nix
+```
+
+To build Nix for the current operating system/architecture use
+
+```console
+$ nix-build
+```
+
+or if you have a flake-enabled nix:
+
+```console
+$ nix build
+```
+
+This will build `defaultPackage` attribute defined in the `flake.nix`
+file. To build for other platforms add one of the following suffixes to
+it: aarch64-linux, i686-linux, x86\_64-darwin, x86\_64-linux. i.e.
+
+```console
+$ nix-build -A defaultPackage.x86_64-linux
+```
+
+To build all dependencies and start a shell in which all environment
+variables are set up so that those dependencies can be found:
+
+```console
+$ nix-shell
+```
+
+To build Nix itself in this shell:
+
+```console
+[nix-shell]$ ./bootstrap.sh
+[nix-shell]$ ./configure $configureFlags --prefix=$(pwd)/outputs/out
+[nix-shell]$ make -j $NIX_BUILD_CORES
+```
+
+To install it in `$(pwd)/outputs` and test it:
+
+```console
+[nix-shell]$ make install
+[nix-shell]$ make installcheck -j $NIX_BUILD_CORES
+[nix-shell]$ ./outputs/out/bin/nix --version
+nix (Nix) 3.0
+```
+
+To run a functional test:
+
+```console
+make tests/test-name-should-auto-complete.sh.test
+```
+
+To run the unit-tests for C++ code:
+
+```
+make check
+```
+
+If you have a flakes-enabled Nix you can replace:
+
+```console
+$ nix-shell
+```
+
+by:
+
+```console
+$ nix develop
+```