Update README.md

#### Use source code

##### Compilation

###### Step 1 Install necessary tools

##### Software requirements

```shell

# Install tools

Centos7 : 

$ yum install gfortran qt4 flex bison 

$ yum install mysql-devel mysql

- Ubuntu 18.04 or higher

- CMake 3.14 or higher

- CUDA 10.0 or higher

- NVIDIA driver 418 or higher

Ubuntu 16.04 or 18.04: 

$ sudo apt-get install gfortran qt4-qmake flex bison 

$ sudo apt-get install libmysqlclient-dev mysql-client

```

Verify the existence of `libmysqlclient_r.so`:

```shell

# Verify existence

$ locate libmysqlclient_r.so

```

##### Compilation

If not, you need to create a symbolic link:

###### Step 1 Install dependencies

```shell

# Locate libmysqlclient.so

$ sudo updatedb

$ locate libmysqlclient.so 

# Create symbolic link

$ sudo ln -s /path/to/libmysqlclient.so /path/to/libmysqlclient_r.so

$ cd [Milvus sourcecode path]/core

./ubuntu_build_deps.sh

```

###### Step 2 Build

When the build is completed, all the stuff that you need in order to run Milvus will be installed under `[Milvus root path]/core/milvus`.

If you encounter the following error message,

`protocol https not supported or disabled in libcurl`

please reinstall CMake with curl:

1. Install curl development files:

   ```shell

   CentOS 7:   

   $ yum install curl-devel

   Ubuntu 16.04 or 18.04: 

   $ sudo apt-get install libcurl4-openssl-dev

   ```

2. Install [CMake 3.14](https://github.com/Kitware/CMake/releases/download/v3.14.6/cmake-3.14.6.tar.gz): 

   ```shell

   $ ./bootstrap --system-curl 

   $ make 

   $ sudo make install

   ```

##### code format and linting

Install clang-format and clang-tidy

```shell

CentOS 7:   

$ yum install clang

Ubuntu 16.04: 

$ sudo apt-get install clang-tidy

$ sudo su

$ wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -

$ apt-add-repository "deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-6.0 main"

$ apt-get update

$ apt-get install clang-format-6.0

Ubuntu 18.04: 

$ sudo apt-get install clang-tidy clang-format

Install clang-format

```shell

$ sudo apt-get install clang-format

$ rm cmake_build/CMakeCache.txt

```

Check code style

```

##### Run code coverage

Install lcov

```shell

CentOS 7:   

$ yum install lcov

Ubuntu 16.04 or 18.04: 

$ sudo apt-get install lcov

```

```shell  

$ ./build.sh -u -c

```

Run mysql docker

Run MySQL docker

```shell 

docker pull mysql:latest

docker run -p 3306:3306 -e MYSQL_ROOT_PASSWORD=123456 -d mysql:latest

```

Run code coverage

```shell  

$ ./coverage.sh -u root -p 123456 -t 127.0.0.1

```

Or start your own MySQL server, and then run code coverage

```shell

$ ./coverage.sh -u ${MYSQL_USERNAME} -p ${MYSQL_PASSWORD} -t ${MYSQL_SERVER_IP}

```

##### Launch Milvus server

```shell

$ pip install pymilvus==0.2.0

```

Create a new file `example.py`, and add [Python example code](https://github.com/milvus-io/pymilvus/blob/branch-0.3.1/examples/AdvancedExample.py) to it.

Create a new file `example.py`, and add [Python example code](https://github.com/milvus-io/pymilvus/blob/master/examples/AdvancedExample.py) to it.

Run the example code.

## Contribution guidelines

Contributions are welcomed and greatly appreciated. If you want to contribute to Milvus, please read our [contribution guidelines](CONTRIBUTING.md). This project adheres to the [code of conduct](CODE OF CONDUCT.md) of Milvus. By participating, you are expected to uphold this code.

Contributions are welcomed and greatly appreciated. If you want to contribute to Milvus, please read our [contribution guidelines](CONTRIBUTING.md). This project adheres to the [code of conduct](CODE_OF_CONDUCT.md) of Milvus. By participating, you are expected to uphold this code.

We use [GitHub issues](https://github.com/milvus-io/milvus/issues) to track issues and bugs. For general questions and public discussions, please join our community.

We use [GitHub issues](https://github.com/milvus-io/milvus/issues/new) to track issues and bugs. For general questions and public discussions, please join our community.

## Join the Milvus community

[Milvus official website](https://www.milvus.io)

[Milvus docs](https://www.milvus.io/docs/en/QuickStart/)

[Milvus docs](https://www.milvus.io/docs/en/userguide/install_milvus/)

[Milvus blog](https://www.milvus.io/blog/)