Updated Preferred Chain (markdown)

Preferred-Chain.md

@@ -57,3 +57,16 @@ acme.sh  --issue -d example.com .....  --server letsencrypt    --preferred-chain
 ```
 
 
+3.  You can also set the default preferred chain for a specified CA(from v3.0.1).
+
+```
+acme.sh --set-default-chain  --preferred-chain "ISRG"   --server  letsencrypt
+```
+
+When you request a cert from letsencrypt in future, it will use the default preferred chain.
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