use tsing debian mirror in China

It's too slow to use debian offical website to create
filesystem. tsinghua mirror is a good local debian pkgs

Signed-off-by: Liu Changcheng <changcheng.liu@aliyun.com>
Signed-off-by: Joel Fernandes (Google) <joelaf@google.com>
diff --git a/README.md b/README.md
index a1aafc2..a81a89d 100644
--- a/README.md
+++ b/README.md
@@ -163,4 +163,10 @@
 -----------------
 1. Installing g++ with `apt-get install g++` fails.
 
-A. Solution: Run `adeb shell apt-get update` after the `adeb prepare` stage.
+Solution: Run `adeb shell apt-get update` after the `adeb prepare` stage.
+
+2. It's too slow to use debootstrap to create debian fs
+
+Solution: Use a local mirror, for example in China you could use
+https://mirror.tuna.tsinghua.edu.cn/debian/ instead of debian official website
+http://deb.debian.org/debian/