update readme.

Author: EthanYuan

contracts/ckb-bitcoin-spv-type-lock/README.md

@@ -48,7 +48,13 @@ A Bitcoin SPV instance contains two types of cells: **SPV info cell** and **SPV
 - Flags
 
   Flags is the last part of `args` and takes up one byte. The highest two bits 
-  are used to indicate different btc chain types. see [btc chain type] for details.
+  are used to indicate different btc chain types. The supported chain types are:
+  
+  - Mainnet
+  - Testnet
+  - Signet
+  
+  see [BTC chain type] for details.
 
 ### Operations