[ci skip] add proxy config docs

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

DOCS.md

@@ -40,10 +40,10 @@ pipeline:
   scp:
     image: appleboy/drone-scp
     host: example.com
-    target: 
+    target:
 +     - /home/deploy/web1
 +     - /home/deploy/web2
-    source: 
+    source:
 +     - release_1.tar.gz
 +     - release_2.tar.gz
 ```
@@ -89,6 +89,24 @@ Remove target folder before copy files and artifacts to target:
 +   rm: true
 ```
 
+Example configuration using ｀SSHProxyCommand｀:
+
+```diff
+pipeline:
+  scp:
+    image: appleboy/drone-scp
+    host:
+      - example1.com
+      - example2.com
+    target: /home/deploy/web
+    source:
+      - release/*.tar.gz
++   proxy_host: 10.130.33.145
++   proxy_user: ubuntu
++   proxy_port: 22
++   proxy_key: ${PROXY_KEY}
+```
+
 Example configuration for success build:
 
 ```diff
@@ -179,3 +197,21 @@ build.author
 
 build.link
 : link the the build results in drone
+
+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