docs: Add devShell guide

Resolves #135

doc/guide/devshell.md

@@ -0,0 +1,48 @@
+---
+slug: devshell
+---
+
+# DevShell
+
+haskell-flake uses the [`shellFor`][shellFor] function to provide a Haskell development shell. `shellFor` in turn uses the standard [`mkShell`][mkShell] function to create a Nix shell environment. The `mkShellArgs` option can be used to pass custom arguments to `mkShell`.
+
+```nix
+{
+  haskellProjects.default = {
+    devShell = {
+      mkShellArgs = {
+        shellHook = ''
+          export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:${pkgs.flint}/lib
+        ''
+      };
+    }
+  }
+}
+```
+
+## Composing devShells
+
+While `mkShellArgs` is a convenient way to extend the Haskell devShell, sometimes you want to compose multiple devShell environments in a way you want.
+
+The devShell of a haskell-flake project is exposed in the `config.haskellProjects.<name>.outputs.devShell` attribute. You can pass this devShell to the `inputsFrom` argument of a [`mkShell`][mkShell] function in order to include the Haskell devShell in another devShell. The same technique can be used to compose devShells created by other flake-parts modules. 
+
+For example, [in haskell-template](https://github.com/srid/haskell-template/blob/master/flake.nix#L96-L102), we create a top-level devShell that merges the devShell of the haskell-flake project, the devShell of [mission-control](https://github.com/Platonic-Systems/mission-control) and the devShell of [flake-root](https://github.com/srid/flake-root) as follows::
+
+```nix
+{
+  devShell = pkgs.mkShell {
+    inputsFrom = [
+      config.haskellProjects.default.outputs.devShell
+      config.flake-root.devShell
+      config.mission-control.devShell
+    ];
+  };
+}
+```
+
+This sort of composition is either impossible or very complex to do with the `mkShellArgs` approach.
+
+
+[shellFor]: https://nixos.org/manual/nixpkgs/unstable/#haskell-shellFor
+[mkShell]: https://nixos.org/manual/nixpkgs/stable/#sec-pkgs-mkShell
+