Add hardware plugins to installation doc (#22732)

Signed-off-by: Michael Goin <mgoin64@gmail.com>
Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>
Co-authored-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/getting_started/installation/README.md

@@ -14,3 +14,16 @@ vLLM supports the following hardware platforms:
 - [Google TPU](google_tpu.md)
 - [Intel Gaudi](intel_gaudi.md)
 - [AWS Neuron](aws_neuron.md)
+
+## Hardware Plugins
+
+The backends below live **outside** the main `vllm` repository and follow the
+[Hardware-Pluggable RFC](../design/plugin_system.md).
+
+| Accelerator | PyPI / package | Repository |
+|-------------|----------------|------------|
+| Ascend NPU | `vllm-ascend` | <https://github.com/vllm-project/vllm-ascend> |
+| Intel Gaudi (HPU) | N/A, install from source | <https://github.com/vllm-project/vllm-gaudi> |
+| MetaX MACA GPU | N/A, install from source | <https://github.com/MetaX-MACA/vLLM-metax> |
+| Rebellions ATOM / REBEL NPU | `vllm-rbln` | <https://github.com/rebellions-sw/vllm-rbln> |
+| IBM Spyre AIU | `vllm-spyre` | <https://github.com/vllm-project/vllm-spyre> |