docs: add IMPROVEMENTS.md and README banner pointing to it

IMPROVEMENTS.md

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+# Improvements in this fork
+
+This branch (`feature/nvidia-gpu-temps`) adds NVIDIA GPU temperature support
+to [`Meliox/PVE-mods`](https://github.com/Meliox/PVE-mods)'s
+`pve-mod-gui-sensors.sh`.
+
+---
+
+## 1. NVIDIA GPU temperature widget (`thermalGpu`)
+
+**File**: `pve-mod-gui-sensors.sh`
+
+Two additions to the install script:
+
+### a) Sensor JSON injection
+
+When `nvidia-smi` is present on the host, inject one entry per GPU into
+`Nodes.pm`'s `$res->{sensorsOutput}` JSON before the existing
+`lm-sensors` payload:
+
+```json
+"GPU0 RTX 3090": {
+  "Adapter": "NVIDIA",
+  "GPU Core": { "temp1_input": 47.0 }
+}
+```
+
+Each GPU appears as `GPU<idx> <model>` (with the `NVIDIA GeForce ` prefix
+stripped). Calls `nvidia-smi --query-gpu=index,name,temperature.gpu` once
+per request.
+
+### b) ExtJS `thermalGpu` widget
+
+Adds a new widget under the StatusView that filters sensor keys starting
+with `GPU` and renders each as `GPU<idx>:&nbsp;<temp>°C` with the
+existing TempHelper (Fahrenheit conversion supported).
+
+Color thresholds:
+- ≥ 85 °C — yellow / bold
+- ≥ 95 °C — red / bold
+
+The widget is only emitted when the install script's GPU detection
+succeeds, so non-NVIDIA hosts behave exactly like upstream.
+
+---
+
+## How to install this fork
+
+```bash
+wget https://raw.githubusercontent.com/svilendotorg/PVE-mods/feature/nvidia-gpu-temps/pve-mod-gui-sensors.sh
+bash pve-mod-gui-sensors.sh install
+```
+
+Then clear your browser cache and reload the Proxmox web UI. The new
+"GPU Thermal State" row appears in the node summary.
+
+To track upstream:
+
+```bash
+git clone https://github.com/svilendotorg/PVE-mods
+cd PVE-mods
+git remote add upstream https://github.com/Meliox/PVE-mods.git
+git fetch upstream
+git rebase upstream/main
+git push --force-with-lease
+```