[ci skip] add proxy config docs

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

DOCS.md

@@ -88,6 +88,25 @@ pipeline:
       - echo world
 ```
 
+Example configuration for execute commands on a remote server using ｀SSHProxyCommand｀:
+
+```diff
+pipeline:
+  ssh:
+    image: appleboy/drone-ssh
+    host: foo.com
+    username: root
+    port: 22
+    key: ${DEPLOY_KEY}
+    script:
+      - echo hello
+      - echo world
++   proxy_host: 10.130.33.145
++   proxy_user: ubuntu
++   proxy_port: 22
++   proxy_key: ${PROXY_KEY}
+```
+
 Example configuration for success build:
 
 ```diff
@@ -151,3 +170,21 @@ timeout
 
 command_timeout
 : Command timeout is the maximum amount of time for the execute commands, default is 60 secs.
+
+proxy_host
+: proxy hostname or IP
+
+proxy_port
+: ssh port of proxy host
+
+proxy_username
+: account for proxy host user
+
+proxy_password
+: password for proxy host user
+
+proxy_key
+: plain text of proxy private key
+
+proxy_key_path
+: key path of proxy private key