docs: rutherford dictates on the RTX 3060 via cuda

The README described the vulkan iGPU rebuild as the only variant.
rutherford decodes on its RTX 3060 through the cuda cargo feature, and
build.sh now takes the feature as its argument, so document both.

README.md

@@ -109,12 +109,14 @@ You also need to install all the dependencies:
 - fcitx5-hangul
 - pipewire-alsa (whisrs captures audio via raw ALSA; without it the default
   PCM is the headset-jack mic, which is silent when nothing is plugged in)
-- a Vulkan driver for your GPU (vulkan-intel + vulkan-icd-loader on Arch)
+- a GPU driver with its toolkit: vulkan-intel + vulkan-icd-loader on the
+  laptop's iGPU, cuda + nvidia on rutherford's RTX 3060
 - whisrs (hold-to-talk voice dictation on SUPER+SHIFT+F23, which is what the
-  Copilot key on newer laptops sends; on Arch install
+  Copilot key on newer laptops sends, on AltGr on rutherford; on Arch install
-  whisrs-git from the AUR). Rebuild it locally with -march=native and the
+  whisrs-git from the AUR). Rebuild it locally with -march=native and a GPU
-  vulkan cargo feature, otherwise decoding is too slow; on this machine the
+  cargo feature (vulkan or cuda — see the GPU feature you have), otherwise
-  rebuild tooling lives in ~/Projects/whisrs-local-build
+  decoding is slow; the rebuild tooling lives in ~/Projects/whisrs-local-build
+  and takes the feature as its argument
 
 There are probably more, I will update the list next time I'm
 installing it on a new computer.