[skip ci] Update C++ SDK readme. (#973)

* Update README.md

Update C++ SDK readme.

* [skip ci] Update C++ SDK readme.

sdk/README.md

+# Milvus C++ SDK
+
+### Get C++ SDK
+
+If you compile Milvus from source, C++ SDK is already in `[Milvus root path]/sdk`. If you install Milvus from Docker images, you need to download the whole `sdk` folder to your host.
+
+### Requirements
+
+CMake 3.14 or higher
+
 ### Build C++ SDK
 
-The C++ SDK source code is under milvus/sdk.
-If you want to build sdk project, follow below steps:
+You must build the C++ SDK before using it:
+
 ```shell
- # build project
+ # build C++ SDK
  $ cd [Milvus root path]/sdk
  $ ./build.sh
 ```
 
 ### Try C++ example
 
-Firstly, you need to start a Milvus server.
-If you've already built the entire milvus project, just start Milvus server with the following command:
-```shell
- # start milvus server
- $ cd [Milvus root path]/core
- $ ./start_server.sh
-```
-You can also use Docker to start Milvus server:
-```shell
- # pull milvus docker image and start milvus server
- $ docker pull milvusdb/milvus:latest
- $ docker run --runtime=nvidia -p 19530:19530 -d milvusdb/milvus:latest
-```
+You must have a running Milvus server to try the C++ example. Refer to [Milvus Documentation](https://milvus.io/docs/guides/get_started/install_milvus/install_milvus.md) to learn how to install and run a Milvus server.
 
 Run C++ example:
 
-```shell
- # run milvus C++ example
+ ```shell
+ # run Milvus C++ example
  $ cd [Milvus root path]/sdk/cmake_build/examples/simple
  $ ./sdk_simple
-```
+ ```
 
-### Make your own C++ client project
+### Create your own C++ client project
 
 Create a folder for the project, and copy C++ SDK header and library files into it.
+
 ```shell
  # create project folder
  $ mkdir MyMilvusClient
@@ -46,8 +45,9 @@ Create a folder for the project, and copy C++ SDK header and library files into
  $ cp [Milvus root path]/sdk/include/Status.h .
 ```
 
-Create file main.cpp in the project folder, and copy the following code into it:
-```shell
+Create file `main.cpp` in the project folder, and copy the following code into it:
+
+```c++
 #include "./MilvusApi.h"
 #include "./Status.h"
 
@@ -64,28 +64,32 @@ int main() {
 }
 ```
 
-Create file CMakeList.txt in the project folder, and copy the following code into it:
-```shell
+Create file `CMakeLists.txt` in the project folder, and copy the following code into it:
+
+```bash
  cmake_minimum_required(VERSION 3.14)
  project(test)
- set(CMAKE_CXX_STANDARD 14)
+ set(CMAKE_CXX_STANDARD 17)
 
  add_executable(milvus_client main.cpp)
  target_link_libraries(milvus_client
-         ${PROJECT_SOURCE_DIR}/libmilvus_sdk.so)
+         ${PROJECT_SOURCE_DIR}/libmilvus_sdk.so
+         pthread)
 ```
 
 Now there are 5 files in your project:
+
 ```shell
 MyMilvusClient
- |-CMakeList.txt
+ |-CMakeLists.txt
  |-main.cpp
  |-libmilvus_sdk.so
  |-MilvusApi.h
  |-Status.h
-  ```
+```
 
 Build the project:
+
 ```shell
  $ mkdir cmake_build
  $ cd cmake_build
@@ -94,6 +98,7 @@ Build the project:
 ```
 
 Run your client program:
+
 ```shell
  $ ./milvus_client
 ```