README.md: update fetch code instructions

Signed-off-by: Andy Hu <andy.hu@starfivetech.com>

Author: andyhu-stf

README.md

@@ -30,8 +30,9 @@ Checkout this repository  (e.g.: branch `JH7110_VisionFive2_devel`). Then checko
 
 	$ git clone https://github.com/starfive-tech/VisionFive2.git
 	$ cd VisionFive2
-	$ git checkout JH7110_VisionFive2_devel
+	$ git checkout --track origin/JH7110_VisionFive2_devel
 	$ git submodule update --init --recursive
+	$ cd linux && git branch JH7110_VisionFive2_devel origin/JH7110_VisionFive2_devel && cd ..
 This will take some time and require around 9GB of disk space. Some modules may fail because certain dependencies don't have the best git hosting. The only solution is to wait and try again later (or ask someone for a copy of that source repository).
 
 For user who build the release tag version, the above command is enough. For developer, need to switch the 5 submodules `buildroot`, `u-boot`, `linux`, `opensbi`, `soft_3rdpart` to correct branch manually, also could refer to `.gitmodule`