Expose experimentalFetchTree in home-manager

This should work around #14, which breaks fetches on newer versions of
Nix. I was hesitant to expose this as Nix changes may break things with
no warning, but it gives users already willing to run newer versions of
Nix a workaround for the fetch failures they're currently getting.

(And the feature itself seems to work fine on current Nix, since CI is
using it...)

README.md

@@ -19,6 +19,10 @@ Doom Emacs and (almost) all module dependencies.
 Not yet tested on macOS. Likely works as long as you do not need an "app" (you
 launch Emacs from the commandline or through other means). I intend to fix this.
 
+You may encounter "Cannot find Git revision" errors on Nix versions newer than
+2.18.x (see #14). Try enabling `experimentalFetchTree` to work around this (see
+below).
+
 Please report any issues.
 
 ## How to use
@@ -177,6 +181,10 @@ support use without flakes.
   For example to include Emacs package `treesit-grammars.with-all-grammars`:
   `extraPackages = epkgs: [ epkgs.treesit-grammars.with-all-grammars ];`.
 
+- `experimentalFetchTree`: fetch packages using `fetchTree`, which is more
+  efficient but considered experimental in Nix (subject to changes which might
+  break fetches).
+
 There are a few other settings but they are not typically useful. See the
 source.