docs: Add VLLM_NIXL in main readme (#23)

README.md

@@ -53,7 +53,8 @@ We provide 3 types of builds:
 
 1. `STANDARD` which includes our default set of backends (onnx, openvino...)
 2. `TENSORRTLLM` which includes our TRT-LLM backend
-3. `VLLM` which includes our VLLM backend
+3. `VLLM` which includes our VLLM backend using NCCL communication library.
+4. `VLLM_NIXL` which includes our VLLM backend using new NIXL communication library.
 
 For example, if you want to build a container for the `STANDARD` backends you can run