完善readme

README.md

 # pyscalebox APIs library
 
+## Prepare
+grpc need the python3
+`python3` can be installed with the following commdand:
+```bash
+yum install python3 python3-pip
+python3 -m pip install --upgrade pip==20.3.4
+python3 -m pip install --upgrade setuptools
+```
 ## Installation
 `pyscalebox` can be installed with the following command:
 
 ```bash
 git clone http://hub.cstcloud.cn/git/csst/pyscalebox.git
 cd pyscalebox
-pip install -r requirements.txt
-python setup.py install
+python3 -m pip install -r requirements.txt
+python3 setup.py install
 ```
 ## Configuration
 - CSST_PIPELINE_GRPC_SERVER = scalebox plat server    # default: 122.96.144.152:39092