Improve docstring of the graphscope module

[wenyuanyu]

Currently the help of module itself does not produce very useful information. I think it is a good idea to improve the doc there, following the example of pandas for example

>>> help(graphscope)
Help on package graphscope:

NAME
    graphscope

DESCRIPTION
    # -*- coding: utf-8 -*-
    #
    # Copyright 2020 Alibaba Group Holding Limited. All Rights Reserved.
    #
    # Licensed under the Apache License, Version 2.0 (the "License");
    # you may not use this file except in compliance with the License.
    # You may obtain a copy of the License at
    #
    #     http://www.apache.org/licenses/LICENSE-2.0
    #
    # Unless required by applicable law or agreed to in writing, software
    # distributed under the License is distributed on an "AS IS" BASIS,
    # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    # See the License for the specific language governing permissions and
    # limitations under the License.
    #

PACKAGE CONTENTS
    analytical (package)
    client (package)
    config
    dataset (package)
    deploy (package)
    experimental (package)
    framework (package)
    interactive (package)
    learning (package)
    proto (package)
    version

DATA
    Vertex = Vertex

VERSION
    0.1.1

FILE
    /opt/conda/lib/python3.8/site-packages/graphscope/__init__.py