Upate README.md for 2.9 release

Signed-off-by: Li, Xun <[email protected]>

Author: llly

README.md

@@ -47,7 +47,7 @@ Build the Intel(R) SGX SDK and Intel(R) SGX PSW Package
   * SUSE Linux Enterprise Server 12 64bits
 
 - Use the following command(s) to install the required tools to build the Intel(R) SGX SDK:  
-  * On Ubuntu 16.04 
+  * On Ubuntu 16.04:
   ```
     $ sudo apt-get install build-essential ocaml automake autoconf libtool wget python libssl-dev git
   ```
@@ -74,6 +74,7 @@ Build the Intel(R) SGX SDK and Intel(R) SGX PSW Package
    ```
     $ sudo yum update binutils
    ```
+   **Note**:  To build Intel(R) SGX SDK, gcc version is required to be 7.3 or above and glibc version is required to be 2.27 or above. For Ubuntu 16.04, Red Hat Enterprise Linux 7.4, CentOS 7.5, Fedora 27, SUSE Linux Enterprise Server 12, you may need to update gcc and glibc version.
 - Use the following command to install additional required tools and latest Intel(R) SGX SDK Installer to build the Intel(R) SGX PSW:  
   1)  To install the additional required tools:
       * On Ubuntu 16.04 and Ubuntu 18.04:
@@ -101,13 +102,18 @@ Build the Intel(R) SGX SDK and Intel(R) SGX PSW Package
   $ ./download_prebuilt.sh
 ```
 
+- Copy the mitigation tools from external/toolset to /usr/local/bin and make sure they have execute permission:
+  ```
+    $ sudo cp external/toolset/{as,ld,ld.gold,objdump} /usr/local/bin
+    $ which as ld ld.gold objdump
+  ```
+
 ### Build the Intel(R) SGX SDK and Intel(R) SGX SDK Installer
 - To build Intel(R) SGX SDK with default configuration, enter the following command:
 ```
   $ make sdk
 ```  
-You can find the tools and libraries generated in the `build/linux` directory.
-  **Note**: You can also go to the `sdk` folder and use the `make` command to build the Intel(R) SGX SDK component only. 
+You can find the three flavors of tools and libraries generated in the `build` directory.
 
 - This repository supports to build the Intel(R) SGX SDK with below three combinations:
   * `USE_OPT_LIBS=0` --- build SDK using SGXSSL and open sourced String/Math
@@ -123,6 +129,7 @@ You can find the tools and libraries generated in the `build/linux` directory.
 ```
   $ make sdk DEBUG=1
 ```
+
 - To clean the files generated by previous `make sdk` command, enter the following command:  
 ```
   $ make clean
@@ -135,7 +142,7 @@ You can find the generated Intel(R) SGX SDK installer ``sgx_linux_x64_sdk_${vers
 
 **Note**: The above command builds the Intel(R) SGX SDK with default configuration firstly and then generates the target SDK Installer. To build the Intel(R) SGX SDK Installer with debug information kept in the tools and libraries, enter the following command:
 ```
-$ make sdk_install_pkg DEBUG=1
+  $ make sdk_install_pkg DEBUG=1
 ```
 
 ### Build the Intel(R) SGX PSW and Intel(R) SGX PSW Installer