docs: update README (#688)

README.md

@@ -48,6 +48,9 @@ pip install flashinfer -i https://flashinfer.ai/whl/cu121/torch2.3/
 git clone https://github.com/sgl-project/sglang.git
 cd sglang
 
+# https://stackoverflow.com/questions/69711606/how-to-install-a-package-using-pip-in-editable-mode-with-pyproject-toml
+python3 -m pip install --upgrade pip
+
 pip install -e "python[all]"
 
 # Install FlashInfer CUDA kernels