nixpkgs docs: move shell section to its own file

doc/functions/shell.section.md

@@ -0,0 +1,22 @@
+---
+title: pkgs.mkShell
+author: zimbatm
+date: 2017-10-30
+---
+
+# mkShell
+
+pkgs.mkShell is a special kind of derivation that is only useful when using
+it combined with nix-shell. It will in fact fail to instantiate when invoked
+with nix-build.
+
+## Usage
+
+```nix
+{ pkgs ? import <nixpkgs> {} }:
+pkgs.mkShell {
+  # this will make all the build inputs from hello and gnutar available to the shell environment
+  inputsFrom = with pkgs; [ hello gnutar ];
+  buildInputs = [ pkgs.gnumake ];
+}
+```