add Proxy setting for using plugin command

Closes #7150

docs/reference/modules/plugins.asciidoc

@@ -142,6 +142,26 @@ bin/plugin --install mobz/elasticsearch-head --timeout 1m
 bin/plugin --install mobz/elasticsearch-head --timeout 0
 -----------------------------------
 
+[float]
+==== Proxy settings
+
+To install a plugin via a proxy, you can pass the proxy details using the environment variables `proxyHost` and `proxyPort`.
+
+On Linux and Mac, here is an example of setting it:
+
+[source,shell]
+-----------------------------------
+bin/plugin -DproxyHost=host_name -DproxyPort=port_number --install mobz/elasticsearch-head
+-----------------------------------
+
+On Windows, here is an example of setting it:
+
+[source,shell]
+-----------------------------------
+set JAVA_OPTS="-DproxyHost=host_name -DproxyPort=port_number"
+bin/plugin --install mobz/elasticsearch-head
+-----------------------------------
+
 [float]
 ==== Lucene version dependent plugins