Add troubleshooting information for INSTALL.md and enhance readability (#1274)

* Update INSTALL.md

1. Add new troubleshooting message;
2. Enhance readability.

Signed-off-by: Lutkin Wang <yamasite@qq.com>

* Update INSTALL.md

Signed-off-by: Lutkin Wang <yamasite@qq.com>

* Update INSTALL.md

Signed-off-by: Lutkin Wang <yamasite@qq.com>

* Update INSTALL.md

Add CentOS link.

Signed-off-by: Lutkin Wang <yamasite@qq.com>

INSTALL.md

@@ -1,21 +1,37 @@
 # Install Milvus from Source Code
 
+<!-- TOC -->
+
 - [Build from source](#build-from-source)
+    - [Requirements](#requirements)
+    - [Compilation](#compilation)
+    - [Launch Milvus server](#launch-milvus-server)
 - [Compile Milvus on Docker](#compile-milvus-on-docker)
+    - [Step 1 Pull Milvus Docker images](#step-1-pull-milvus-docker-images)
+    - [Step 2 Start the Docker container](#step-2-start-the-docker-container)
+    - [Step 3 Download Milvus source code](#step-3-download-milvus-source-code)
+    - [Step 4 Compile Milvus in the container](#step-4-compile-milvus-in-the-container)
 - [Troubleshooting](#troubleshooting)
+    - [Error message: `protocol https not supported or disabled in libcurl`](#error-message-protocol-https-not-supported-or-disabled-in-libcurl)
+    - [Error message: `internal compiler error`](#error-message-internal-compiler-error)
+    - [Error message: `error while loading shared libraries: libmysqlpp.so.3`](#error-message-error-while-loading-shared-libraries-libmysqlppso3)
+    - [CMake version is not supported](#cmake-version-is-not-supported)
+
+<!-- /TOC -->
 
 ## Build from source
 
 ### Requirements
 
 - Ubuntu 18.04 or higher
+- CentOS 7
 
-  If your operating system is not Ubuntu 18.04 or higher, we recommend you to pull a [docker image of Ubuntu 18.04](https://docs.docker.com/install/linux/docker-ce/ubuntu/) as your compilation environment.
+If your operating system does not meet the requirements, we recommend that you pull a Docker image of [Ubuntu 18.04](https://docs.docker.com/install/linux/docker-ce/ubuntu/) or [CentOS 7](https://docs.docker.com/install/linux/docker-ce/centos/) as your compilation environment.
   
-- GCC 7.0 or higher to support C++17
+- GCC 7.0 or higher to support C++ 17
 - CMake 3.12 or higher
 
-##### For GPU-enabled version, you will also need:
+For GPU-enabled version, you will also need:
 
 - CUDA 10.0 or higher
 - NVIDIA driver 418 or higher
@@ -31,19 +47,34 @@ $ ./ubuntu_build_deps.sh
 
 #### Step 2 Build
 
+##### Build in Ubuntu
+
 ```shell
 $ cd [Milvus root path]/core
 $ ./build.sh -t Debug
+```
+
 or 
+
+```shell
 $ ./build.sh -t Release
 ```
 
-By default, it will build CPU-only version. To build GPU version, add `-g` option
+##### Build in CentOS
+
+```shell
+$ cd [Milvus root path]/core
+$ ./centos7_build_deps.sh
+```
+
+By default, it will build CPU-only version. To build GPU version, add `-g` option.
+
 ```shell
 $ ./build.sh -g
 ```
 
-If you want to know the complete build options, run
+If you want to know the complete build options, run the following command.
+
 ```shell
 $./build.sh -h
 ```
@@ -147,29 +178,47 @@ $ ./start_server.sh
 
 ## Troubleshooting
 
-1. If you encounter the following error when compiling: 
-`protocol https not supported or disabled in libcurl`.
-First, make sure you have `libcurl4-openssl-dev` installed in your system.
-Then try reinstalling the latest CMake from source with `--system-curl` option:
+### Error message: `protocol https not supported or disabled in libcurl`
+
+Follow the steps below to solve this problem:
+
+1. Make sure you have `libcurl4-openssl-dev` installed in your system.
+2. Try reinstalling the latest CMake from source with `--system-curl` option:
 
    ```shell
    $ ./bootstrap --system-curl
    $ make
    $ sudo make install
    ```
-    If the `--system-curl` command doesn't work, you can also reinstall CMake in **Ubuntu Software** on your local computer.
 
-2. If you encounter the following error when compiling in a Docker image: `internal compiler error`. Try increasing the memory allocated to docker first.
+   If the `--system-curl` command doesn't work, you can also reinstall CMake in **Ubuntu Software** on your local computer.
 
-3. If you encounter the following error during compilation:
+### Error message: `internal compiler error`
 
-    ```
-    .../bin/milvus_server: error while loading shared libraries: libmysqlpp.so.3: cannot open shared object file: No such file or directory
+Try increasing the memory allocated to docker.
+
+### Error message: `error while loading shared libraries: libmysqlpp.so.3`
+
+Follow the steps below to solve this problem:
+    
+1. Check whether `libmysqlpp.so.3` is correctly installed.
+2. If `libmysqlpp.so.3` is installed, check whether it is added to `LD_LIBRARY_PATH`.
+
+### CMake version is not supported
+
+Follow the steps below to install a supported version of CMake:
+
+1. Remove the unsupported version of CMake.
+2. Get CMake 3.12 or higher. Here we get CMake 3.12.
+
+    ```shell
+    $ wget https://cmake.org/files/v3.12/cmake-3.12.2-Linux-x86_64.tar.gz
     ```
 
-    Try the following solutions:
-    
-    1. Check whether `libmysqlpp.so.3` is correctly installed;
-    2. If `libmysqlpp.so.3` is installed, check whether it is added to `LD_LIBRARY_PATH`.
-
+3. Extract the file and install CMake.
 
+    ```shell
+    $ tar zxvf cmake-3.12.2-Linux-x86_64.tar.gz
+    $ mv cmake-3.12.2-Linux-x86_64 /opt/cmake-3.12.2
+    $ ln -sf /opt/cmake-3.12.2/bin/* /usr/bin/
+    ```