readme: Add instructions on how to get SPKI

ReadMe.md

@@ -657,6 +657,13 @@ https://github.com/pymumu/smartdns/releases
     bind [::]:6053 -no-speed-check -group office -no-rule-addr
     ```
 
+1. DOT的SPKI如何获取  
+    SPKI可以通过DNS服务商发布的页面获取，如果没有发布，可以通过如下命令获取，其中将对应IP地址更换为要获取SPKI的IP地址。
+
+    ```sh
+    echo | openssl s_client -connect '1.0.0.1:853' 2>/dev/null | openssl x509 -pubkey -noout | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64
+    ```
+
 ## 编译
 
 smartdns包含了编译软件包的脚本，支持编译luci，debian，openwrt，optare安装包，可执行`package/build-pkg.sh`编译。

ReadMe_en.md

@@ -638,7 +638,7 @@ Note: Merlin firmware is derived from ASUS firmware and can theoretically be use
     Enable cache serve expired feature with `serve-expired yes` to improve the cache hit rate and reduce the CPU consumption.
     This feature will return TTL = 0 to the client after the TTL timeout, and send a new query request again at the same time, and cache the new results for later query.
 
-1. How does the second DNS customize more behavior?
+1. How does the second DNS customize more behavior?  
     The second DNS can be used as the upstream of other DNS servers to provide more query behaviors. Bind configuration support can bind multiple ports. Different ports can be set with different flags to implement different functions, such as
 
     ```sh
@@ -646,6 +646,13 @@ Note: Merlin firmware is derived from ASUS firmware and can theoretically be use
     bind [::]:6053 -no-speed-check -group office -no-rule-addr
     ```
 
+1. How to get SPKI of DOT  
+    The SPKI can be obtained from the page published by the DNS service provider. If it is not published, it can be obtained by the following command, replace IP with your own IP.
+
+    ````sh
+    echo | openssl s_client -connect '1.0.0.1:853' 2>/dev/null | openssl x509 -pubkey -noout | openssl pkey -pubin -outform der | openssl dgst -sha256 -binary | openssl enc -base64
+    ````
+
 ## Compile
 
 smartdns contains scripts for compiling packages, supports compiling luci, debian, openwrt, opare installation packages, and can execute `package/build-pkg.sh` compilation.