Updated Preferred Chain (markdown)

Preferred-Chain.md

@@ -1 +1,34 @@
 Using `--preferred-chain` to select alternate chain.
+
+If the ACME CA provides multiple cert chain, you can use `--preferred-chain` to select one. Otherwise, it will get the default chain.
+
+1. For Letsencrpt.org Staging Server:
+
+    There are 2 chains provided:
+
+    |   Name            |  Default  |
+    |-------------------|-----------|
+    |   Fake LE Root X1 | Yes       |
+    |   Fake LE Root X2 |   No     |
+
+    You select the ca like:
+```
+acme.sh  --issue -d example.com .....     --preferred-chain  "Fake LE Root X2"
+```
+
+2. For Letsencrypt.org Production server:
+
+    There are 2 chains provided:
+
+    |   Name            |  Default  |
+    |-------------------|-----------|
+    |   DST Root CA X3 | Yes       |
+    |   ISRG Root X1 |   No     |
+
+    You select the ca like:
+```
+acme.sh  --issue -d example.com .....     --preferred-chain  "ISRG Root X1"
+```
+
+
+